Network Working Group M. Crispin
Request for Comments: 2060 University of Washington
Obsoletes: 1730 December 1996
Category: Standards Track
INTERNET MESSAGE Access PROTOCOL - VERSION 4rev1
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1)
allows a client to access and manipulate electronic mail messages on
a server. IMAP4rev1 permits manipulation of remote message folders,
called "mailboxes", in a way that is functionally equivalent to local
mailboxes. IMAP4rev1 also provides the capability for an offline
client to resynchronize with the server (see also [IMAP-DISC]).
IMAP4rev1 includes operations for creating, deleting, and renaming
mailboxes; checking for new messages; permanently removing messages;
setting and clearing flags; [RFC-822] and [MIME-IMB] parsing;
searching; and selective fetching of message attributes, texts, and
portions thereof. Messages in IMAP4rev1 are accessed by the use of
numbers. These numbers are either message sequence numbers or unique
identifiers.
IMAP4rev1 supports a single server. A mechanism for accessing
configuration information to support multiple IMAP4rev1 servers is
discussed in [ACAP].
IMAP4rev1 does not specify a means of posting mail; this function is
handled by a mail transfer protocol sUCh as [SMTP].
IMAP4rev1 is designed to be upwards compatible from the [IMAP2] and
unpublished IMAP2bis protocols. In the course of the evolution of
IMAP4rev1, some ASPects in the earlier protocol have become obsolete.
Obsolete commands, responses, and data formats which an IMAP4rev1
implementation may encounter when used with an earlier implementation
are described in [IMAP-OBSOLETE].
Other compatibility issues with IMAP2bis, the most common variant of
the earlier protocol, are discussed in [IMAP-COMPAT]. A full
discussion of compatibility issues with rare (and presumed extinct)
variants of [IMAP2] is in [IMAP-HISTORICAL]; this document is
primarily of historical interest.
Table of Contents
IMAP4rev1 Protocol Specification .................................. 4
1. How to Read This Document ................................. 4
1.1. Organization of This Document ............................. 4
1.2. Conventions Used in This Document ......................... 4
2. Protocol Overview ......................................... 5
2.1. Link Level ................................................ 5
2.2. Commands and Responses .................................... 6
2.2.1. Client Protocol Sender and Server Protocol Receiver ....... 6
2.2.2. Server Protocol Sender and Client Protocol Receiver ....... 7
2.3. Message Attributes ........................................ 7
2.3.1. Message Numbers ........................................... 7
2.3.1.1. Unique Identifier (UID) Message Attribute ......... 7
2.3.1.2. Message Sequence Number Message Attribute ......... 9
2.3.2. Flags Message Attribute .................................... 9
2.3.3. Internal Date Message Attribute ........................... 10
2.3.4. [RFC-822] Size Message Attribute .......................... 11
2.3.5. Envelope Structure Message Attribute ...................... 11
2.3.6. Body Structure Message Attribute .......................... 11
2.4. Message Texts ............................................. 11
3. State and Flow Diagram .................................... 11
3.1. Non-Authenticated State ................................... 11
3.2. Authenticated State ....................................... 11
3.3. Selected State ............................................ 12
3.4. Logout State .............................................. 12
4. Data Formats .............................................. 12
4.1. Atom ...................................................... 13
4.2. Number .................................................... 13
4.3. String ..................................................... 13
4.3.1. 8-bit and Binary Strings .................................. 13
4.4. Parenthesized List ........................................ 14
4.5. NIL ....................................................... 14
5. Operational Considerations ................................ 14
5.1. Mailbox Naming ............................................ 14
5.1.1. Mailbox Hierarchy Naming .................................. 14
5.1.2. Mailbox Namespace Naming Convention ....................... 14
5.1.3. Mailbox International Naming Convention ................... 15
5.2. Mailbox Size and Message Status Updates ................... 16
5.3. Response when no Command in Progress ...................... 16
5.4. Autologout Timer .......................................... 16
5.5. Multiple Commands in Progress ............................. 17
6. Client Commands ........................................... 17
6.1. Client Commands - Any State ............................... 18
6.1.1. CAPABILITY Command ........................................ 18
6.1.2. NOOP Command .............................................. 19
6.1.3. LOGOUT Command ............................................ 20
6.2. Client Commands - Non-Authenticated State ................. 20
6.2.1. AUTHENTICATE Command ...................................... 21
6.2.2. LOGIN Command ............................................. 22
6.3. Client Commands - Authenticated State ..................... 22
6.3.1. SELECT Command ............................................ 23
6.3.2. EXAMINE Command ........................................... 24
6.3.3. CREATE Command ............................................ 25
6.3.4. DELETE Command ............................................ 26
6.3.5. RENAME Command ............................................ 27
6.3.6. SUBSCRIBE Command ......................................... 29
6.3.7. UNSUBSCRIBE Command ....................................... 30
6.3.8. LIST Command .............................................. 30
6.3.9. LSUB Command .............................................. 32
6.3.10. STATUS Command ............................................ 33
6.3.11. APPEND Command ............................................ 34
6.4. Client Commands - Selected State .......................... 35
6.4.1. CHECK Command ............................................. 36
6.4.2. CLOSE Command ............................................. 36
6.4.3. EXPUNGE Command ........................................... 37
6.4.4. SEARCH Command ............................................ 37
6.4.5. FETCH Command ............................................. 41
6.4.6. STORE Command ............................................. 45
6.4.7. COPY Command .............................................. 46
6.4.8. UID Command ............................................... 47
6.5. Client Commands - Experimental/Expansion .................. 48
6.5.1. X<atom> Command ........................................... 48
7. Server Responses .......................................... 48
7.1. Server Responses - Status Responses ....................... 49
7.1.1. OK Response ............................................... 51
7.1.2. NO Response ............................................... 51
7.1.3. BAD Response .............................................. 52
7.1.4. PREAUTH Response .......................................... 52
7.1.5. BYE Response .............................................. 52
7.2. Server Responses - Server and Mailbox Status .............. 53
7.2.1. CAPABILITY Response ....................................... 53
7.2.2. LIST Response .............................................. 54
7.2.3. LSUB Response ............................................. 55
7.2.4 STATUS Response ........................................... 55
7.2.5. SEARCH Response ........................................... 55
7.2.6. FLAGS Response ............................................ 56
7.3. Server Responses - Mailbox Size ........................... 56
7.3.1. EXISTS Response ........................................... 56
7.3.2. RECENT Response ........................................... 57
7.4. Server Responses - Message Status ......................... 57
7.4.1. EXPUNGE Response .......................................... 57
7.4.2. FETCH Response ............................................ 58
7.5. Server Responses - Command Continuation Request ........... 63
8. Sample IMAP4rev1 connection ............................... 63
9. Formal Syntax ............................................. 64
10. Author"s Note ............................................. 74
11. Security Considerations ................................... 74
12. Author"s Address .......................................... 75
Appendices ........................................................ 76
A. References ................................................ 76
B. Changes from RFC1730 ..................................... 77
C. Key Word Index ............................................ 79
IMAP4rev1 Protocol Specification
1. How to Read This Document
1.1. Organization of This Document
This document is written from the point of view of the implementor of
an IMAP4rev1 client or server. Beyond the protocol overview in
section 2, it is not optimized for someone trying to understand the
operation of the protocol. The material in sections 3 through 5
provides the general context and definitions with which IMAP4rev1
operates.
Sections 6, 7, and 9 describe the IMAP commands, responses, and
syntax, respectively. The relationships among these are such that it
is almost impossible to understand any of them separately. In
particular, do not attempt to deduce command syntax from the command
section alone; instead refer to the Formal Syntax section.
1.2. Conventions Used in This Document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The following terms are used in this document to signify the
requirements of this specification.
1) MUST, or the adjective REQUIRED, means that the definition is
an absolute requirement of the specification.
2) MUST NOT that the definition is an absolute prohibition of the
specification.
3) SHOULD means that there may exist valid reasons in particular
circumstances to ignore a particular item, but the full
implications MUST be understood and carefully weighed before
choosing a different course.
4) SHOULD NOT means that there may exist valid reasons in
particular circumstances when the particular behavior is
acceptable or even useful, but the full implications SHOULD be
understood and the case carefully weighed before implementing
any behavior described with this label.
5) MAY, or the adjective OPTIONAL, means that an item is truly
optional. One vendor may choose to include the item because a
particular marketplace requires it or because the vendor feels
that it enhances the product while another vendor may omit the
same item. An implementation which does not include a
particular option MUST be prepared to interoperate with another
implementation which does include the option.
"Can" is used instead of "may" when referring to a possible
circumstance or situation, as opposed to an optional facility of
the protocol.
"User" is used to refer to a human user, whereas "client" refers
to the software being run by the user.
"Connection" refers to the entire sequence of client/server
interaction from the initial establishment of the network
connection until its termination. "Session" refers to the
sequence of client/server interaction from the time that a mailbox
is selected (SELECT or EXAMINE command) until the time that
selection ends (SELECT or EXAMINE of another mailbox, CLOSE
command, or connection termination).
Characters are 7-bit US-ASCII unless otherwise specified. Other
character sets are indicated using a "CHARSET", as described in
[MIME-IMT] and defined in [CHARSET]. CHARSETs have important
additional semantics in addition to defining character set; refer
to these documents for more detail.
2. Protocol Overview
2.1. Link Level
The IMAP4rev1 protocol assumes a reliable data stream such as
provided by TCP. When TCP is used, an IMAP4rev1 server listens on
port 143.
2.2. Commands and Responses
An IMAP4rev1 connection consists of the establishment of a
client/server network connection, an initial greeting from the
server, and client/server interactions. These client/server
interactions consist of a client command, server data, and a server
completion result response.
All interactions transmitted by client and server are in the form of
lines; that is, strings that end with a CRLF. The protocol receiver
of an IMAP4rev1 client or server is either reading a line, or is
reading a sequence of octets with a known count followed by a line.
2.2.1. Client Protocol Sender and Server Protocol Receiver
The client command begins an operation. Each client command is
prefixed with an identifier (typically a short alphanumeric string,
e.g. A0001, A0002, etc.) called a "tag". A different tag is
generated by the client for each command.
There are two cases in which a line from the client does not
represent a complete command. In one case, a command argument is
quoted with an octet count (see the description of literal in String
under Data Formats); in the other case, the command arguments require
server feedback (see the AUTHENTICATE command). In either case, the
server sends a command continuation request response if it is ready
for the octets (if appropriate) and the remainder of the command.
This response is prefixed with the token "+".
Note: If, instead, the server detected an error in the command, it
sends a BAD completion response with tag matching the command (as
described below) to reject the command and prevent the client from
sending any more of the command.
It is also possible for the server to send a completion response
for some other command (if multiple commands are in progress), or
untagged data. In either case, the command continuation request
is still pending; the client takes the appropriate action for the
response, and reads another response from the server. In all
cases, the client MUST send a complete command (including
receiving all command continuation request responses and command
continuations for the command) before initiating a new command.
The protocol receiver of an IMAP4rev1 server reads a command line
from the client, parses the command and its arguments, and transmits
server data and a server command completion result response.
2.2.2. Server Protocol Sender and Client Protocol Receiver
Data transmitted by the server to the client and status responses
that do not indicate command completion are prefixed with the token
"*", and are called untagged responses.
Server data MAY be sent as a result of a client command, or MAY be
sent unilaterally by the server. There is no syntactic difference
between server data that resulted from a specific command and server
data that were sent unilaterally.
The server completion result response indicates the success or
failure of the operation. It is tagged with the same tag as the
client command which began the operation. Thus, if more than one
command is in progress, the tag in a server completion response
identifies the command to which the response applies. There are
three possible server completion responses: OK (indicating success),
NO (indicating failure), or BAD (indicating protocol error such as
unrecognized command or command syntax error).
The protocol receiver of an IMAP4rev1 client reads a response line
from the server. It then takes action on the response based upon the
first token of the response, which can be a tag, a "*", or a "+".
A client MUST be prepared to accept any server response at all times.
This includes server data that was not requested. Server data SHOULD
be recorded, so that the client can reference its recorded copy
rather than sending a command to the server to request the data. In
the case of certain server data, the data MUST be recorded.
This topic is discussed in greater detail in the Server Responses
section.
2.3. Message Attributes
In addition to message text, each message has several attributes
associated with it. These attributes may be retrieved individually
or in conjunction with other attributes or message texts.
2.3.1. Message Numbers
Messages in IMAP4rev1 are accessed by one of two numbers; the unique
identifier and the message sequence number.
2.3.1.1. Unique Identifier (UID) Message Attribute
A 32-bit value assigned to each message, which when used with the
unique identifier validity value (see below) forms a 64-bit value
that is permanently guaranteed not to refer to any other message in
the mailbox. Unique identifiers are assigned in a strictly ascending
fashion in the mailbox; as each message is added to the mailbox it is
assigned a higher UID than the message(s) which were added
previously.
Unlike message sequence numbers, unique identifiers are not
necessarily contiguous. Unique identifiers also persist across
sessions. This permits a client to resynchronize its state from a
previous session with the server (e.g. disconnected or offline access
clients); this is discussed further in [IMAP-DISC].
Associated with every mailbox is a unique identifier validity value,
which is sent in an UIDVALIDITY response code in an OK untagged
response at mailbox selection time. If unique identifiers from an
earlier session fail to persist to this session, the unique
identifier validity value MUST be greater than the one used in the
earlier session.
Note: Unique identifiers MUST be strictly ascending in the mailbox
at all times. If the physical message store is re-ordered by a
non-IMAP agent, this requires that the unique identifiers in the
mailbox be regenerated, since the former unique identifers are no
longer strictly ascending as a result of the re-ordering. Another
instance in which unique identifiers are regenerated is if the
message store has no mechanism to store unique identifiers.
Although this specification recognizes that this may be
unavoidable in certain server environments, it STRONGLY ENCOURAGES
message store implementation techniques that avoid this problem.
Another cause of non-persistance is if the mailbox is deleted and
a new mailbox with the same name is created at a later date, Since
the name is the same, a client may not know that this is a new
mailbox unless the unique identifier validity is different. A
good value to use for the unique identifier validity value is a
32-bit representation of the creation date/time of the mailbox.
It is alright to use a constant such as 1, but only if it
guaranteed that unique identifiers will never be reused, even in
the case of a mailbox being deleted (or renamed) and a new mailbox
by the same name created at some future time.
The unique identifier of a message MUST NOT change during the
session, and SHOULD NOT change between sessions. However, if it is
not possible to preserve the unique identifier of a message in a
subsequent session, each subsequent session MUST have a new unique
identifier validity value that is larger than any that was used
previously.
2.3.1.2. Message Sequence Number Message Attribute
A relative position from 1 to the number of messages in the mailbox.
This position MUST be ordered by ascending unique identifier. As
each new message is added, it is assigned a message sequence number
that is 1 higher than the number of messages in the mailbox before
that new message was added.
Message sequence numbers can be reassigned during the session. For
example, when a message is permanently removed (expunged) from the
mailbox, the message sequence number for all subsequent messages is
decremented. Similarly, a new message can be assigned a message
sequence number that was once held by some other message prior to an
expunge.
In addition to accessing messages by relative position in the
mailbox, message sequence numbers can be used in mathematical
calculations. For example, if an untagged "EXISTS 11" is received,
and previously an untagged "8 EXISTS" was received, three new
messages have arrived with message sequence numbers of 9, 10, and 11.
Another example; if message 287 in a 523 message mailbox has UID
12345, there are exactly 286 messages which have lesser UIDs and 236
messages which have greater UIDs.
2.3.2. Flags Message Attribute
A list of zero or more named tokens associated with the message. A
flag is set by its addition to this list, and is cleared by its
removal. There are two types of flags in IMAP4rev1. A flag of
either type may be permanent or session-only.
A system flag is a flag name that is pre-defined in this
specification. All system flags begin with "". Certain system
flags (Deleted and Seen) have special semantics described
elsewhere. The currently-defined system flags are:
Seen Message has been read
Answered Message has been answered
Flagged Message is "flagged" for urgent/special attention
Deleted Message is "deleted" for removal by later EXPUNGE
Draft Message has not completed composition (marked as a
draft).
Recent Message is "recently" arrived in this mailbox. This
session is the first session to have been notified
about this message; subsequent sessions will not see
Recent set for this message. This flag can not be
altered by the client.
If it is not possible to determine whether or not
this session is the first session to be notified
about a message, then that message SHOULD be
considered recent.
If multiple connections have the same mailbox
selected simultaneously, it is undefined which of
these connections will see newly-arrives messages
with Recent set and which will see it without
Recent set.
A keyword is defined by the server implementation. Keywords do
not begin with "". Servers MAY permit the client to define new
keywords in the mailbox (see the description of the
PERMANENTFLAGS response code for more information).
A flag may be permanent or session-only on a per-flag basis.
Permanent flags are those which the client can add or remove
from the message flags permanently; that is, subsequent sessions
will see any change in permanent flags. Changes to session
flags are valid only in that session.
Note: The Recent system flag is a special case of a
session flag. Recent can not be used as an argument in a
STORE command, and thus can not be changed at all.
2.3.3. Internal Date Message Attribute
The internal date and time of the message on the server. This is not
the date and time in the [RFC-822] header, but rather a date and time
which reflects when the message was received. In the case of
messages delivered via [SMTP], this SHOULD be the date and time of
final delivery of the message as defined by [SMTP]. In the case of
messages delivered by the IMAP4rev1 COPY command, this SHOULD be the
internal date and time of the source message. In the case of
messages delivered by the IMAP4rev1 APPEND command, this SHOULD be
the date and time as specified in the APPEND command description.
All other cases are implementation defined.
2.3.4. [RFC-822] Size Message Attribute
The number of octets in the message, as expressed in [RFC-822]
format.
2.3.5. Envelope Structure Message Attribute
A parsed representation of the [RFC-822] envelope information (not to
be confused with an [SMTP] envelope) of the message.
2.3.6. Body Structure Message Attribute
A parsed representation of the [MIME-IMB] body structure information
of the message.
2.4. Message Texts
In addition to being able to fetch the full [RFC-822] text of a
message, IMAP4rev1 permits the fetching of portions of the full
message text. Specifically, it is possible to fetch the [RFC-822]
message header, [RFC-822] message body, a [MIME-IMB] body part, or a
[MIME-IMB] header.
3. State and Flow Diagram
An IMAP4rev1 server is in one of four states. Most commands are
valid in only certain states. It is a protocol error for the client
to attempt a command while the command is in an inappropriate state.
In this case, a server will respond with a BAD or NO (depending upon
server implementation) command completion result.
3.1. Non-Authenticated State
In non-authenticated state, the client MUST supply authentication
credentials before most commands will be permitted. This state is
entered when a connection starts unless the connection has been pre-
authenticated.
3.2. Authenticated State
In authenticated state, the client is authenticated and MUST select a
mailbox to access before commands that affect messages will be
permitted. This state is entered when a pre-authenticated connection
starts, when acceptable authentication credentials have been
provided, or after an error in selecting a mailbox.
3.3. Selected State
In selected state, a mailbox has been selected to access. This state
is entered when a mailbox has been successfully selected.
3.4. Logout State
In logout state, the connection is being terminated, and the server
will close the connection. This state can be entered as a result of
a client request or by unilateral server decision.
+--------------------------------------+
initial connection and server greeting
+--------------------------------------+
(1) (2) (3)
VV
+-----------------+
non-authenticated
+-----------------+
(7) (4)
VV VV
+----------------+
authenticated <=++
+----------------+
(7) (5) (6)
VV
+--------+
selected==++
+--------+
(7)
VV VV VV VV
+--------------------------------------+
logout and close connection
+--------------------------------------+
(1) connection without pre-authentication (OK greeting)
(2) pre-authenticated connection (PREAUTH greeting)
(3) rejected connection (BYE greeting)
(4) successful LOGIN or AUTHENTICATE command
(5) successful SELECT or EXAMINE command
(6) CLOSE command, or failed SELECT or EXAMINE command
(7) LOGOUT command, server shutdown, or connection closed
4. Data Formats
IMAP4rev1 uses textual commands and responses. Data in IMAP4rev1 can
be in one of several forms: atom, number, string, parenthesized list,
or NIL.
4.1. Atom
An atom consists of one or more non-special characters.
4.2. Number
A number consists of one or more digit characters, and represents a
numeric value.
4.3. String
A string is in one of two forms: literal and quoted string. The
literal form is the general form of string. The quoted string form
is an alternative that avoids the overhead of processing a literal at
the cost of limitations of characters that can be used in a quoted
string.
A literal is a sequence of zero or more octets (including CR and LF),
prefix-quoted with an octet count in the form of an open brace ("{"),
the number of octets, close brace ("}"), and CRLF. In the case of
literals transmitted from server to client, the CRLF is immediately
followed by the octet data. In the case of literals transmitted from
client to server, the client MUST wait to receive a command
continuation request (described later in this document) before
sending the octet data (and the remainder of the command).
A quoted string is a sequence of zero or more 7-bit characters,
excluding CR and LF, with double quote (<">) characters at each end.
The empty string is represented as either "" (a quoted string with
zero characters between double quotes) or as {0} followed by CRLF (a
literal with an octet count of 0).
Note: Even if the octet count is 0, a client transmitting a
literal MUST wait to receive a command continuation request.
4.3.1. 8-bit and Binary Strings
8-bit textual and binary mail is supported through the use of a
[MIME-IMB] content transfer encoding. IMAP4rev1 implementations MAY
transmit 8-bit or multi-octet characters in literals, but SHOULD do
so only when the [CHARSET] is identified.
Although a BINARY body encoding is defined, unencoded binary strings
are not permitted. A "binary string" is any string with NUL
characters. Implementations MUST encode binary data into a textual
form such as BASE64 before transmitting the data. A string with an
excessive amount of CTL characters MAY also be considered to be
binary.
4.4. Parenthesized List
Data structures are represented as a "parenthesized list"; a sequence
of data items, delimited by space, and bounded at each end by
parentheses. A parenthesized list can contain other parenthesized
lists, using multiple levels of parentheses to indicate nesting.
The empty list is represented as () -- a parenthesized list with no
members.
4.5. NIL
The special atom "NIL" represents the non-existence of a particular
data item that is represented as a string or parenthesized list, as
distinct from the empty string "" or the empty parenthesized list ().
5. Operational Considerations
5.1. Mailbox Naming
The interpretation of mailbox names is implementation-dependent.
However, the case-insensitive mailbox name INBOX is a special name
reserved to mean "the primary mailbox for this user on this server".
5.1.1. Mailbox Hierarchy Naming
If it is desired to export hierarchical mailbox names, mailbox names
MUST be left-to-right hierarchical using a single character to
separate levels of hierarchy. The same hierarchy separator character
is used for all levels of hierarchy within a single name.
5.1.2. Mailbox Namespace Naming Convention
By convention, the first hierarchical element of any mailbox name
which begins with "#" identifies the "namespace" of the remainder of
the name. This makes it possible to disambiguate between different
types of mailbox stores, each of which have their own namespaces.
For example, implementations which offer access to USENET
newsgroups MAY use the "#news" namespace to partition the USENET
newsgroup namespace from that of other mailboxes. Thus, the
comp.mail.misc newsgroup would have an mailbox name of
"#news.comp.mail.misc", and the name "comp.mail.misc" could refer
to a different object (e.g. a user"s private mailbox).
5.1.3. Mailbox International Naming Convention
By convention, international mailbox names are specified using a
modified version of the UTF-7 encoding described in [UTF-7]. The
purpose of these modifications is to correct the following problems
with UTF-7:
1) UTF-7 uses the "+" character for shifting; this conflicts with
the common use of "+" in mailbox names, in particular USENET
newsgroup names.
2) UTF-7"s encoding is BASE64 which uses the "/" character; this
conflicts with the use of "/" as a popular hierarchy delimiter.
3) UTF-7 prohibits the unencoded usage of ""; this conflicts with
the use of "" as a popular hierarchy delimiter.
4) UTF-7 prohibits the unencoded usage of "~"; this conflicts with
the use of "~" in some servers as a home Directory indicator.
5) UTF-7 permits multiple alternate forms to represent the same
string; in particular, printable US-ASCII chararacters can be
represented in encoded form.
In modified UTF-7, printable US-ASCII characters except for "&"
represent themselves; that is, characters with octet values 0x20-0x25
and 0x27-0x7e. The character "&" (0x26) is represented by the two-
octet sequence "&-".
All other characters (octet values 0x00-0x1f, 0x7f-0xff, and all
Unicode 16-bit octets) are represented in modified BASE64, with a
further modification from [UTF-7] that "," is used instead of "/".
Modified BASE64 MUST NOT be used to represent any printing US-ASCII
character which can represent itself.
"&" is used to shift to modified BASE64 and "-" to shift back to US-
ASCII. All names start in US-ASCII, and MUST end in US-ASCII (that
is, a name that ends with a Unicode 16-bit octet MUST end with a "-
").
For example, here is a mailbox name which mixes English, Japanese,
and Chinese text: ~peter/mail/&ZeVnLIqe-/&U,BTFw-
5.2. Mailbox Size and Message Status Updates
At any time, a server can send data that the client did not request.
Sometimes, such behavior is REQUIRED. For example, agents other than
the server MAY add messages to the mailbox (e.g. new mail delivery),
change the flags of message in the mailbox (e.g. simultaneous access
to the same mailbox by multiple agents), or even remove messages from
the mailbox. A server MUST send mailbox size updates automatically
if a mailbox size change is observed during the processing of a
command. A server SHOULD send message flag updates automatically,
without requiring the client to request such updates explicitly.
Special rules exist for server notification of a client about the
removal of messages to prevent synchronization errors; see the
description of the EXPUNGE response for more detail.
Regardless of what implementation decisions a client makes on
remembering data from the server, a client implementation MUST record
mailbox size updates. It MUST NOT assume that any command after
initial mailbox selection will return the size of the mailbox.
5.3. Response when no Command in Progress
Server implementations are permitted to send an untagged response
(except for EXPUNGE) while there is no command in progress. Server
implementations that send such responses MUST deal with flow control
considerations. Specifically, they MUST either (1) verify that the
size of the data does not exceed the underlying transport"s available
window size, or (2) use non-blocking writes.
5.4. Autologout Timer
If a server has an inactivity autologout timer, that timer MUST be of
at least 30 minutes" duration. The receipt of ANY command from the
client during that interval SHOULD suffice to reset the autologout
timer.
5.5. Multiple Commands in Progress
The client MAY send another command without waiting for the
completion result response of a command, subject to ambiguity rules
(see below) and flow control constraints on the underlying data
stream. Similarly, a server MAY begin processing another command
before processing the current command to completion, subject to
ambiguity rules. However, any command continuation request responses
and command continuations MUST be negotiated before any subsequent
command is initiated.
The exception is if an ambiguity would result because of a command
that would affect the results of other commands. Clients MUST NOT
send multiple commands without waiting if an ambiguity would result.
If the server detects a possible ambiguity, it MUST execute commands
to completion in the order given by the client.
The most obvious example of ambiguity is when a command would affect
the results of another command; for example, a FETCH of a message"s
flags and a STORE of that same message"s flags.
A non-obvious ambiguity occurs with commands that permit an untagged
EXPUNGE response (commands other than FETCH, STORE, and SEARCH),
since an untagged EXPUNGE response can invalidate sequence numbers in
a subsequent command. This is not a problem for FETCH, STORE, or
SEARCH commands because servers are prohibited from sending EXPUNGE
responses while any of those commands are in progress. Therefore, if
the client sends any command other than FETCH, STORE, or SEARCH, it
MUST wait for a response before sending a command with message
sequence numbers.
For example, the following non-waiting command sequences are invalid:
FETCH + NOOP + STORE
STORE + COPY + FETCH
COPY + COPY
CHECK + FETCH
The following are examples of valid non-waiting command sequences:
FETCH + STORE + SEARCH + CHECK
STORE + COPY + EXPUNGE
6. Client Commands
IMAP4rev1 commands are described in this section. Commands are
organized by the state in which the command is permitted. Commands
which are permitted in multiple states are listed in the minimum
permitted state (for example, commands valid in authenticated and
selected state are listed in the authenticated state commands).
Command arguments, identified by "Arguments:" in the command
descriptions below, are described by function, not by syntax. The
precise syntax of command arguments is described in the Formal Syntax
section.
Some commands cause specific server responses to be returned; these
are identified by "Responses:" in the command descriptions below.
See the response descriptions in the Responses section for
information on these responses, and the Formal Syntax section for the
precise syntax of these responses. It is possible for server data to
be transmitted as a result of any command; thus, commands that do not
specifically require server data specify "no specific responses for
this command" instead of "none".
The "Result:" in the command description refers to the possible
tagged status responses to a command, and any special interpretation
of these status responses.
6.1. Client Commands - Any State
The following commands are valid in any state: CAPABILITY, NOOP, and
LOGOUT.
6.1.1. CAPABILITY Command
Arguments: none
Responses: REQUIRED untagged response: CAPABILITY
Result: OK - capability completed
BAD - command unknown or arguments invalid
The CAPABILITY command requests a listing of capabilities that the
server supports. The server MUST send a single untagged
CAPABILITY response with "IMAP4rev1" as one of the listed
capabilities before the (tagged) OK response. This listing of
capabilities is not dependent upon connection state or user. It
is therefore not necessary to issue a CAPABILITY command more than
once in a connection.
A capability name which begins with "AUTH=" indicates that the
server supports that particular authentication mechanism. All
such names are, by definition, part of this specification. For
example, the authorization capability for an experimental
"blurdybloop" authenticator would be "AUTH=XBLURDYBLOOP" and not
"XAUTH=BLURDYBLOOP" or "XAUTH=XBLURDYBLOOP".
Other capability names refer to extensions, revisions, or
amendments to this specification. See the documentation of the
CAPABILITY response for additional information. No capabilities,
beyond the base IMAP4rev1 set defined in this specification, are
enabled without explicit client action to invoke the capability.
See the section entitled "Client Commands -
Experimental/Expansion" for information about the form of site or
implementation-specific capabilities.
Example: C: abcd CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=KERBEROS_V4
S: abcd OK CAPABILITY completed
6.1.2. NOOP Command
Arguments: none
Responses: no specific responses for this command (but see below)
Result: OK - noop completed
BAD - command unknown or arguments invalid
The NOOP command always succeeds. It does nothing.
Since any command can return a status update as untagged data, the
NOOP command can be used as a periodic poll for new messages or
message status updates during a period of inactivity. The NOOP
command can also be used to reset any inactivity autologout timer
on the server.
Example: C: a002 NOOP
S: a002 OK NOOP completed
. . .
C: a047 NOOP
S: * 22 EXPUNGE
S: * 23 EXISTS
S: * 3 RECENT
S: * 14 FETCH (FLAGS (Seen Deleted))
S: a047 OK NOOP completed
6.1.3. LOGOUT Command
Arguments: none
Responses: REQUIRED untagged response: BYE
Result: OK - logout completed
BAD - command unknown or arguments invalid
The LOGOUT command informs the server that the client is done with
the connection. The server MUST send a BYE untagged response
before the (tagged) OK response, and then close the network
connection.
Example: C: A023 LOGOUT
S: * BYE IMAP4rev1 Server logging out
S: A023 OK LOGOUT completed
(Server and client then close the connection)
6.2. Client Commands - Non-Authenticated State
In non-authenticated state, the AUTHENTICATE or LOGIN command
establishes authentication and enter authenticated state. The
AUTHENTICATE command provides a general mechanism for a variety of
authentication techniques, whereas the LOGIN command uses the
traditional user name and plaintext password pair.
Server implementations MAY allow non-authenticated access to certain
mailboxes. The convention is to use a LOGIN command with the userid
"anonymous". A password is REQUIRED. It is implementation-dependent
what requirements, if any, are placed on the password and what access
restrictions are placed on anonymous users.
Once authenticated (including as anonymous), it is not possible to
re-enter non-authenticated state.
In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
the following commands are valid in non-authenticated state:
AUTHENTICATE and LOGIN.
6.2.1. AUTHENTICATE Command
Arguments: authentication mechanism name
Responses: continuation data can be requested
Result: OK - authenticate completed, now in authenticated state
NO - authenticate failure: unsupported authentication
mechanism, credentials rejected
BAD - command unknown or arguments invalid,
authentication exchange cancelled
The AUTHENTICATE command indicates an authentication mechanism,
such as described in [IMAP-AUTH], to the server. If the server
supports the requested authentication mechanism, it performs an
authentication protocol exchange to authenticate and identify the
client. It MAY also negotiate an OPTIONAL protection mechanism
for subsequent protocol interactions. If the requested
authentication mechanism is not supported, the server SHOULD
reject the AUTHENTICATE command by sending a tagged NO response.
The authentication protocol exchange consists of a series of
server challenges and client answers that are specific to the
authentication mechanism. A server challenge consists of a
command continuation request response with the "+" token followed
by a BASE64 encoded string. The client answer consists of a line
consisting of a BASE64 encoded string. If the client wishes to
cancel an authentication exchange, it issues a line with a single
"*". If the server receives such an answer, it MUST reject the
AUTHENTICATE command by sending a tagged BAD response.
A protection mechanism provides integrity and privacy protection
to the connection. If a protection mechanism is negotiated, it is
applied to all subsequent data sent over the connection. The
protection mechanism takes effect immediately following the CRLF
that concludes the authentication exchange for the client, and the
CRLF of the tagged OK response for the server. Once the
protection mechanism is in effect, the stream of command and
response octets is processed into buffers of ciphertext. Each
buffer is transferred over the connection as a stream of octets
prepended with a four octet field in network byte order that
represents the length of the following data. The maximum
ciphertext buffer length is defined by the protection mechanism.
Authentication mechanisms are OPTIONAL. Protection mechanisms are
also OPTIONAL; an authentication mechanism MAY be implemented
without any protection mechanism. If an AUTHENTICATE command
fails with a NO response, the client MAY try another
authentication mechanism by issuing another AUTHENTICATE command,
or MAY attempt to authenticate by using the LOGIN command. In
other words, the client MAY request authentication types in
decreasing order of preference, with the LOGIN command as a last
resort.
Example: S: * OK KerberosV4 IMAP4rev1 Server
C: A001 AUTHENTICATE KERBEROS_V4
S: + AmFYig==
C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
+nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
S: + or//EoAADZI=
C: DiAF5A4gA+oOIALuBkAAmw==
S: A001 OK Kerberos V4 authentication successful
Note: the line breaks in the first client answer are for editorial
clarity and are not in real authenticators.
6.2.2. LOGIN Command
Arguments: user name
password
Responses: no specific responses for this command
Result: OK - login completed, now in authenticated state
NO - login failure: user name or password rejected
BAD - command unknown or arguments invalid
The LOGIN command identifies the client to the server and carries
the plaintext password authenticating this user.
Example: C: a001 LOGIN SMITH SESAME
S: a001 OK LOGIN completed
6.3. Client Commands - Authenticated State
In authenticated state, commands that manipulate mailboxes as atomic
entities are permitted. Of these commands, the SELECT and EXAMINE
commands will select a mailbox for access and enter selected state.
In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
the following commands are valid in authenticated state: SELECT,
EXAMINE, CREATE, DELETE, RENAME, SUBSCRIBE, UNSUBSCRIBE, LIST, LSUB,
STATUS, and APPEND.
6.3.1. SELECT Command
Arguments: mailbox name
Responses: REQUIRED untagged responses: FLAGS, EXISTS, RECENT
OPTIONAL OK untagged responses: UNSEEN, PERMANENTFLAGS
Result: OK - select completed, now in selected state
NO - select failure, now in authenticated state: no
such mailbox, can"t access mailbox
BAD - command unknown or arguments invalid
The SELECT command selects a mailbox so that messages in the
mailbox can be accessed. Before returning an OK to the client,
the server MUST send the following untagged data to the client:
FLAGS Defined flags in the mailbox. See the description
of the FLAGS response for more detail.
<n> EXISTS The number of messages in the mailbox. See the
description of the EXISTS response for more detail.
<n> RECENT The number of messages with the Recent flag set.
See the description of the RECENT response for more
detail.
OK [UIDVALIDITY <n>]
The unique identifier validity value. See the
description of the UID command for more detail.
to define the initial state of the mailbox at the client.
The server SHOULD also send an UNSEEN response code in an OK
untagged response, indicating the message sequence number of the
first unseen message in the mailbox.
If the client can not change the permanent state of one or more of
the flags listed in the FLAGS untagged response, the server SHOULD
send a PERMANENTFLAGS response code in an OK untagged response,
listing the flags that the client can change permanently.
Only one mailbox can be selected at a time in a connection;
simultaneous access to multiple mailboxes requires multiple
connections. The SELECT command automatically deselects any
currently selected mailbox before attempting the new selection.
Consequently, if a mailbox is selected and a SELECT command that
fails is attempted, no mailbox is selected.
If the client is permitted to modify the mailbox, the server
SHOULD prefix the text of the tagged OK response with the
"[READ-WRITE]" response code.
If the client is not permitted to modify the mailbox but is
permitted read access, the mailbox is selected as read-only, and
the server MUST prefix the text of the tagged OK response to
SELECT with the "[READ-ONLY]" response code. Read-only access
through SELECT differs from the EXAMINE command in that certain
read-only mailboxes MAY permit the change of permanent state on a
per-user (as opposed to global) basis. Netnews messages marked in
a server-based .newsrc file are an example of such per-user
permanent state that can be modified with read-only mailboxes.
Example: C: A142 SELECT INBOX
S: * 172 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 12] Message 12 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * FLAGS (Answered Flagged Deleted Seen Draft)
S: * OK [PERMANENTFLAGS (Deleted Seen *)] Limited
S: A142 OK [READ-WRITE] SELECT completed
6.3.2. EXAMINE Command
Arguments: mailbox name
Responses: REQUIRED untagged responses: FLAGS, EXISTS, RECENT
OPTIONAL OK untagged responses: UNSEEN, PERMANENTFLAGS
Result: OK - examine completed, now in selected state
NO - examine failure, now in authenticated state: no
such mailbox, can"t access mailbox
BAD - command unknown or arguments invalid
The EXAMINE command is identical to SELECT and returns the same
output; however, the selected mailbox is identified as read-only.
No changes to the permanent state of the mailbox, including
per-user state, are permitted.
The text of the tagged OK response to the EXAMINE command MUST
begin with the "[READ-ONLY]" response code.
Example: C: A932 EXAMINE blurdybloop
S: * 17 EXISTS
S: * 2 RECENT
S: * OK [UNSEEN 8] Message 8 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * FLAGS (Answered Flagged Deleted Seen Draft)
S: * OK [PERMANENTFLAGS ()] No permanent flags permitted
S: A932 OK [READ-ONLY] EXAMINE completed
6.3.3. CREATE Command
Arguments: mailbox name
Responses: no specific responses for this command
Result: OK - create completed
NO - create failure: can"t create mailbox with that name
BAD - command unknown or arguments invalid
The CREATE command creates a mailbox with the given name. An OK
response is returned only if a new mailbox with that name has been
created. It is an error to attempt to create INBOX or a mailbox
with a name that refers to an extant mailbox. Any error in
creation will return a tagged NO response.
If the mailbox name is suffixed with the server"s hierarchy
separator character (as returned from the server by a LIST
command), this is a declaration that the client intends to create
mailbox names under this name in the hierarchy. Server
implementations that do not require this declaration MUST ignore
it.
If the server"s hierarchy separator character appears elsewhere in
the name, the server SHOULD create any superior hierarchical names
that are needed for the CREATE command to complete successfully.
In other words, an attempt to create "foo/bar/zap" on a server in
which "/" is the hierarchy separator character SHOULD create foo/
and foo/bar/ if they do not already exist.
If a new mailbox is created with the same name as a mailbox which
was deleted, its unique identifiers MUST be greater than any
unique identifiers used in the previous incarnation of the mailbox
UNLESS the new incarnation has a different unique identifier
validity value. See the description of the UID command for more
detail.
Example: C: A003 CREATE owatagusiam/
S: A003 OK CREATE completed
C: A004 CREATE owatagusiam/blurdybloop
S: A004 OK CREATE completed
Note: the interpretation of this example depends on whether "/"
was returned as the hierarchy separator from LIST. If "/" is the
hierarchy separator, a new level of hierarchy named "owatagusiam"
with a member called "blurdybloop" is created. Otherwise, two
mailboxes at the same hierarchy level are created.
6.3.4. DELETE Command
Arguments: mailbox name
Responses: no specific responses for this command
Result: OK - delete completed
NO - delete failure: can"t delete mailbox with that name
BAD - command unknown or arguments invalid
The DELETE command permanently removes the mailbox with the given
name. A tagged OK response is returned only if the mailbox has
been deleted. It is an error to attempt to delete INBOX or a
mailbox name that does not exist.
The DELETE command MUST NOT remove inferior hierarchical names.
For example, if a mailbox "foo" has an inferior "foo.bar"
(assuming "." is the hierarchy delimiter character), removing
"foo" MUST NOT remove "foo.bar". It is an error to attempt to
delete a name that has inferior hierarchical names and also has
the Noselect mailbox name attribute (see the description of the
LIST response for more details).
It is permitted to delete a name that has inferior hierarchical
names and does not have the Noselect mailbox name attribute. In
this case, all messages in that mailbox are removed, and the name
will acquire the Noselect mailbox name attribute.
The value of the highest-used unique identifier of the deleted
mailbox MUST be preserved so that a new mailbox created with the
same name will not reuse the identifiers of the former
incarnation, UNLESS the new incarnation has a different unique
identifier validity value. See the description of the UID command
for more detail.
Examples: C: A682 LIST "" *
S: * LIST () "/" blurdybloop
S: * LIST (Noselect) "/" foo
S: * LIST () "/" foo/bar
S: A682 OK LIST completed
C: A683 DELETE blurdybloop
S: A683 OK DELETE completed
C: A684 DELETE foo
S: A684 NO Name "foo" has inferior hierarchical names
C: A685 DELETE foo/bar
S: A685 OK DELETE Completed
C: A686 LIST "" *
S: * LIST (Noselect) "/" foo
S: A686 OK LIST completed
C: A687 DELETE foo
S: A687 OK DELETE Completed
C: A82 LIST "" *
S: * LIST () "." blurdybloop
S: * LIST () "." foo
S: * LIST () "." foo.bar
S: A82 OK LIST completed
C: A83 DELETE blurdybloop
S: A83 OK DELETE completed
C: A84 DELETE foo
S: A84 OK DELETE Completed
C: A85 LIST "" *
S: * LIST () "." foo.bar
S: A85 OK LIST completed
C: A86 LIST "" %
S: * LIST (Noselect) "." foo
S: A86 OK LIST completed
6.3.5. RENAME Command
Arguments: existing mailbox name
new mailbox name
Responses: no specific responses for this command
Result: OK - rename completed
NO - rename failure: can"t rename mailbox with that name,
can"t rename to mailbox with that name
BAD - command unknown or arguments invalid
The RENAME command changes the name of a mailbox. A tagged OK
response is returned only if the mailbox has been renamed. It is
an error to attempt to rename from a mailbox name that does not
exist or to a mailbox name that already exists. Any error in
renaming will return a tagged NO response.
If the name has inferior hierarchical names, then the inferior
hierarchical names MUST also be renamed. For example, a rename of
"foo" to "zap" will rename "foo/bar" (assuming "/" is the
hierarchy delimiter character) to "zap/bar".
The value of the highest-used unique identifier of the old mailbox
name MUST be preserved so that a new mailbox created with the same
name will not reuse the identifiers of the former incarnation,
UNLESS the new incarnation has a different unique identifier
validity value. See the description of the UID command for more
detail.
Renaming INBOX is permitted, and has special behavior. It moves
all messages in INBOX to a new mailbox with the given name,
leaving INBOX empty. If the server implementation supports
inferior hierarchical names of INBOX, these are unaffected by a
rename of INBOX.
Examples: C: A682 LIST "" *
S: * LIST () "/" blurdybloop
S: * LIST (Noselect) "/" foo
S: * LIST () "/" foo/bar
S: A682 OK LIST completed
C: A683 RENAME blurdybloop sarasoop
S: A683 OK RENAME completed
C: A684 RENAME foo zowie
S: A684 OK RENAME Completed
C: A685 LIST "" *
S: * LIST () "/" sarasoop
S: * LIST (Noselect) "/" zowie
S: * LIST () "/" zowie/bar
S: A685 OK LIST completed
C: Z432 LIST "" *
S: * LIST () "." INBOX
S: * LIST () "." INBOX.bar
S: Z432 OK LIST completed
C: Z433 RENAME INBOX old-mail
S: Z433 OK RENAME completed
C: Z434 LIST "" *
S: * LIST () "." INBOX
S: * LIST () "." INBOX.bar
S: * LIST () "." old-mail
S: Z434 OK LIST completed
6.3.6. SUBSCRIBE Command
Arguments: mailbox
Responses: no specific responses for this command
Result: OK - subscribe completed
NO - subscribe failure: can"t subscribe to that name
BAD - command unknown or arguments invalid
The SUBSCRIBE command adds the specified mailbox name to the
server"s set of "active" or "subscribed" mailboxes as returned by
the LSUB command. This command returns a tagged OK response only
if the subscription is successful.
A server MAY validate the mailbox argument to SUBSCRIBE to verify
that it exists. However, it MUST NOT unilaterally remove an
existing mailbox name from the subscription list even if a mailbox
by that name no longer exists.
Note: this requirement is because some server sites may routinely
remove a mailbox with a well-known name (e.g. "system-alerts")
after its contents expire, with the intention of recreating it
when new contents are appropriate.
Example: C: A002 SUBSCRIBE #news.comp.mail.mime
S: A002 OK SUBSCRIBE completed
6.3.7. UNSUBSCRIBE Command
Arguments: mailbox name
Responses: no specific responses for this command
Result: OK - unsubscribe completed
NO - unsubscribe failure: can"t unsubscribe that name
BAD - command unknown or arguments invalid
The UNSUBSCRIBE command removes the specified mailbox name from
the server"s set of "active" or "subscribed" mailboxes as returned
by the LSUB command. This command returns a tagged OK response
only if the unsubscription is successful.
Example: C: A002 UNSUBSCRIBE #news.comp.mail.mime
S: A002 OK UNSUBSCRIBE completed
6.3..8. LIST Command
Arguments: reference name
mailbox name with possible wildcards
Responses: untagged responses: LIST
Result: OK - list completed
NO - list failure: can"t list that reference or name
BAD - command unknown or arguments invalid
The LIST command returns a subset of names from the complete set
of all names available to the client. Zero or more untagged LIST
replies are returned, containing the name attributes, hierarchy
delimiter, and name; see the description of the LIST reply for
more detail.
The LIST command SHOULD return its data quickly, without undue
delay. For example, it SHOULD NOT go to excess trouble to
calculate Marked or Unmarked status or perform other processing;
if each name requires 1 second of processing, then a list of 1200
names would take 20 minutes!
An empty ("" string) reference name argument indicates that the
mailbox name is interpreted as by SELECT. The returned mailbox
names MUST match the supplied mailbox name pattern. A non-empty
reference name argument is the name of a mailbox or a level of
mailbox hierarchy, and indicates a context in which the mailbox
name is interpreted in an implementation-defined manner.
An empty ("" string) mailbox name argument is a special request to
return the hierarchy delimiter and the root name of the name given
in the reference. The value returned as the root MAY be null if
the reference is non-rooted or is null. In all cases, the
hierarchy delimiter is returned. This permits a client to get the
hierarchy delimiter even when no mailboxes by that name currently
exist.
The reference and mailbox name arguments are interpreted, in an
implementation-dependent fashion, into a canonical form that
represents an unambiguous left-to-right hierarchy. The returned
mailbox names will be in the interpreted form.
Any part of the reference argument that is included in the
interpreted form SHOULD prefix the interpreted form. It SHOULD
also be in the same form as the reference name argument. This
rule permits the client to determine if the returned mailbox name
is in the context of the reference argument, or if something about
the mailbox argument overrode the reference argument. Without
this rule, the client would have to have knowledge of the server"s
naming semantics including what characters are "breakouts" that
override a naming context.
For example, here are some examples of how references and mailbox
names might be interpreted on a UNIX-based server:
Reference Mailbox Name Interpretation
------------ ------------ --------------
~smith/Mail/ foo.* ~smith/Mail/foo.*
archive/ % archive/%
#news. comp.mail.* #news.comp.mail.*
~smith/Mail/ /usr/doc/foo /usr/doc/foo
archive/ ~fred/Mail/* ~fred/Mail/*
The first three examples demonstrate interpretations in the
context of the reference argument. Note that "~smith/Mail" SHOULD
NOT be transformed into something like "/u2/users/smith/Mail", or
it would be impossible for the client to determine that the
interpretation was in the c