Network Working GroupG. VaudreuilRequest for Comments: 1893 Octel Network ServicesCategory: Standards Track January 1996 Enhanced Mail System Status CodesStatus 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.1. Overview There currently is not a standard mechanism for the reporting of mail system errors except for the limited set offered by SMTP and the system specific text descriptions sent in mail messages. There is a pressing need for a rich machine readable status code for use in delivery status notifications [DSN]. This document proposes a new set of status codes for this purpose. SMTP [SMTP] error codes have historically been used for reporting mail system errors. Because of limitations in the SMTP code design, these are not suitable for use in delivery status notifications. SMTP provides about 12 useful codes for delivery reports. The majority of the codes are protocol specific response codes sUCh as the 354 response to the SMTP data command. Each of the 12 useful codes are each overloaded to indicate several error conditions each. SMTP suffers some scars from history, most notably the unfortunate damage to the reply code extension mechanism by uncontrolled use. This proposal facilitates future extensibility by requiring the client to interpret unknown error codes according to the theory of codes while requiring servers to register new response codes. The SMTP theory of reply codes partitioned in the number space such a manner that the remaining available codes will not provide the space needed. The most critical example is the existence of only 5 remaining codes for mail system errors. The mail system classification includes both host and mailbox error conditions. The remaining third digit space would be completely consumed as needed to indicate MIME and media conversion errors and security system errors. A revision to the SMTP theory of reply codes to better distribute the error conditions in the number space will necessarily be incompatible with SMTP. Further, consumption of the remaining reply-code number space for delivery notification reporting will reduce the available codes for new ESMTP extensions. The following proposal is based on the SMTP theory of reply codes. It adopts the success, permanent error, and transient error semantics of the first value, with a further description and classification in
the second. This proposal re-distributes the classifications to better distribute the error conditions, such as separating mailbox from host errors.2. Status Codes This document defines a new set of status codes to report mail system conditions. These status codes are intended to be used for media and language independent status reporting. They are not intended for system specific diagnostics. The syntax of the new status codes is defined as:status-code = class "." subject "." detailclass = "2"/"4"/"5"subject = 1*3digitdetail = 1*3digit White-space characters and comments are NOT allowed within a status- code. Each numeric sub-code within the status-code MUST be eXPressed without leading zero digits. Status codes consist of three numerical fields separated by ".". The first sub-code indicates whether the delivery attempt was successful. The second sub-code indicates the probable source of any delivery anomalies, and the third sub-code indicates a precise error condition. The codes space defined is intended to be extensible only by standards track documents. Mail system specific status codes should be mapped as close as possible to the standard status codes. Servers should send only defined, registered status codes. System specific errors and diagnostics should be carried by means other than status codes. New subject and detail codes will be added over time. Because the number space is large, it is not intended that published status codes will ever be redefined or eliminated. Clients should preserve the extensibility of the code space by reporting the general error described in the subject sub-code when the specific detail is unrecognized. The class sub-code provides a broad classification of the status. The enumerated values the class are defined as: 2.X.X Success Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery. 4.X.X Persistent Transient Failure A persistent transient failure is one in which the message as sent is valid, but some temporary event prevents the successful sending of the message. Sending in the future may be successful. 5.X.X Permanent Failure A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery. A client must recognize and report class sub-code even where subsequent subject sub-codes are unrecognized. The subject sub-code classifies the status. This value applies to each of the three classifications. The subject sub-code, if
recognized, must be reported even if the additional detail provided by the detail sub-code is not recognized. The enumerated values for the subject sub-code are: X.0.X Other or Undefined StatusThere is no additional subject information available. X.1.X Addressing StatusThe address status reports on the originator or destinationaddress. It may include address syntax or validity. Theseerrors can generally be corrected by the sender and retried. X.2.X Mailbox StatusMailbox status indicates that something having to do with themailbox has cause this DSN. Mailbox issues are assumed to beunder the general control of the recipient. X.3.X Mail System StatusMail system status indicates that something having to dowith the destination system has caused this DSN. Systemissues are assumed to be under the general control of thedestination system administrator. X.4.X Network and Routing StatusThe networking or routing codes report status about thedelivery system itself. These system components include anynecessary infrastructure such as Directory and routingservices. Network issues are assumed to be under thecontrol of the destination or intermediate systemadministrator. X.5.X Mail Delivery Protocol StatusThe mail delivery protocol status codes report failuresinvolving the message delivery protocol. These failuresinclude the full range of problems resulting fromimplementation errors or an unreliable connection. Maildelivery protocol issues may be controlled by many partiesincluding the originating system, destination system, orintermediate system administrators. X.6.X Message Content or Media StatusThe message content or media status codes report failuresinvolving the content of the message. These codes reportfailures due to translation, transcoding, or otherwiseunsupported message media. Message content or media issuesare under the control of both the sender and the receiver,both of whom must support a common set of supportedcontent-types. X.7.X Security or Policy StatusThe security or policy status codes report failuresinvolving policies such as per-recipient or per-hostfiltering and cryptographic operations. Security and policystatus issues are assumed to be under the control of eitheror both the sender and recipient. Both the sender andrecipient must permit the exchange of messages and arrange
the exchange of necessary keys and certificates forcryptographic operations.3. Enumerated Status Codes The following section defines and describes the detail sub-code. The detail value provides more information about the status and is defined relative to the subject of the status. 3.1 Other or Undefined Status X.0.0 Other undefined StatusOther undefined status is the only undefined error code. Itshould be used for all errors for which only the class of theerror is known. 3.2 Address Status X.1.0 Other address statusSomething about the address specified in the message causedthis DSN. X.1.1 Bad destination mailbox addressThe mailbox specified in the address does not exist. ForInternet mail names, this means the address portion to theleft of the "@" sign is invalid. This code is only usefulfor permanent failures. X.1.2 Bad destination system addressThe destination system specified in the address does notexist or is incapable of accepting mail. For Internet mailnames, this means the address portion to the right of the"@" is invalid for mail. This codes is only useful forpermanent failures. X.1.3 Bad destination mailbox address syntaxThe destination address was syntactically invalid. This canapply to any field in the address. This code is only usefulfor permanent failures. X.1.4 Destination mailbox address ambiguousThe mailbox address as specified matches one or morerecipients on the destination system. This may result if aheuristic address mapping algorithm is used to map thespecified address to a local mailbox name. X.1.5 Destination address validThis mailbox address as specified was valid. This statuscode should be used for positive delivery reports. X.1.6 Destination mailbox has moved, No forwarding addressThe mailbox address provided was at one time valid, but mailis no longer being accepted for that address. This code isonly useful for permanent failures. X.1.7 Bad sender"s mailbox address syntaxThe sender"s address was syntactically invalid. This canapply to any field in the address. X.1.8 Bad sender"s system addressThe sender"s system specified in the address does not existor is incapable of accepting return mail. For domain names,this means the address portion to the right of the "@" isinvalid for mail. 3.3 Mailbox Status X.2.0 Other or undefined mailbox statusThe mailbox exists, but something about the destinationmailbox has caused the sending of this DSN.
X.2.1 Mailbox disabled, not accepting messagesThe mailbox exists, but is not accepting messages. This maybe a permanent error if the mailbox will never be re-enabledor a transient error if the mailbox is only temporarilydisabled. X.2.2 Mailbox fullThe mailbox is full because the user has exceeded aper-mailbox administrative quota or physical capacity. Thegeneral semantics implies that the recipient can deletemessages to make more space available. This code should beused as a persistent transient failure. X.2.3 Message length exceeds administrative limitA per-mailbox administrative message length limit has beenexceeded. This status code should be used when theper-mailbox message length limit is less than the generalsystem limit. This code should be used as a permanentfailure. X.2.4 Mailing list expansion problemThe mailbox is a mailing list address and the mailing listwas unable to be expanded. This code may represent apermanent failure or a persistent transient failure. 3.4 Mail system status X.3.0 Other or undefined mail system statusThe destination system exists and normally accepts mail, butsomething about the system has caused the generation of thisDSN. X.3.1 Mail system fullMail system storage has been exceeded. The generalsemantics imply that the individual recipient may not beable to delete material to make room for additionalmessages. This is useful only as a persistent transienterror. X.3.2 System not accepting network messagesThe host on which the mailbox is resident is not acceptingmessages. Examples of such conditions include an immanentshutdown, excessive load, or system maintenance. This isuseful for both permanent and permanent transient errors. X.3.3 System not capable of selected featuresSelected features specified for the message are notsupported by the destination system. This can occur ingateways when features from one domain cannot be mapped ontothe supported feature in another. X.3.4 Message too big for systemThe message is larger than per-message size limit. Thislimit may either be for physical or administrative reasons.This is useful only as a permanent error. X.3.5 System incorrectly configuredThe system is not configured in a manner which will permitit to accept this message. 3.5 Network and Routing Status X.4.0 Other or undefined network or routing statusSomething went wrong with the networking, but it is not
clear what the problem is, or the problem cannot be wellexpressed with any of the other provided detail codes. X.4.1 No answer from hostThe outbound connection attempt was not answered, eitherbecause the remote system was busy, or otherwise unable totake a call. This is useful only as a persistent transienterror. X.4.2 Bad connectionThe outbound connection was established, but was otherwiseunable to complete the message transaction, either becauseof time-out, or inadequate connection quality. This isuseful only as a persistent transient error. X.4.3 Directory server failureThe network system was unable to forward the message,because a directory server was unavailable. This is usefulonly as a persistent transient error.The inability to connect to an Internet DNS server is oneexample of the directory server failure error. X.4.4 Unable to routeThe mail system was unable to determine the next hop for themessage because the necessary routing information wasunavailable from the directory server. This is useful forboth permanent and persistent transient errors.A DNS lookup returning only an SOA (Start of Administration)record for a domain name is one example of the unable toroute error. X.4.5 Mail system congestionThe mail system was unable to deliver the message becausethe mail system was congested. This is useful only as apersistent transient error. X.4.6 Routing loop detectedA routing loop caused the message to be forwarded too manytimes, either because of incorrect routing tables or a userforwarding loop. This is useful only as a persistenttransient error. X.4.7 Delivery time expiredThe message was considered too old by the rejecting system,either because it remained on that host too long or becausethe time-to-live value specified by the sender of themessage was exceeded. If possible, the code for the actualproblem found when delivery was attempted should be returnedrather than this code. This is useful only as a persistenttransient error. 3.6 Mail Delivery Protocol Status X.5.0 Other or undefined protocol statusSomething was wrong with the protocol necessary to deliverthe message to the next hop and the problem cannot be wellexpressed with any of the other provided detail codes. X.5.1 Invalid commandA mail transaction protocol command was issued which waseither out of sequence or unsupported. This is useful onlyas a permanent error.
X.5.2 Syntax errorA mail transaction protocol command was issued which couldnot be interpreted, either because the syntax was wrong orthe command is unrecognized. This is useful only as apermanent error. X.5.3 Too many recipientsMore recipients were specified for the message than couldhave been delivered by the protocol. This error shouldnormally result in the segmentation of the message into two,the remainder of the recipients to be delivered on asubsequent delivery attempt. It is included in this list inthe event that such segmentation is not possible. X.5.4 Invalid command argumentsA valid mail transaction protocol command was issued withinvalid arguments, either because the arguments were out ofrange or represented unrecognized features. This is usefulonly as a permanent error. X.5.5 Wrong protocol versionA protocol version mis-match existed which could not beautomatically resolved by the communicating parties. 3.7 Message Content or Message Media Status X.6.0 Other or undefined media errorSomething about the content of a message caused it to beconsidered undeliverable and the problem cannot be wellexpressed with any of the other provided detail codes. X.6.1 Media not supportedThe media of the message is not supported by either thedelivery protocol or the next system in the forwarding path.This is useful only as a permanent error. X.6.2 Conversion required and prohibitedThe content of the message must be converted before it canbe delivered and such conversion is not permitted. Suchprohibitions may be the expression of the sender in themessage itself or the policy of the sending host. X.6.3 Conversion required but not supportedThe message content must be converted to be forwarded butsuch conversion is not possible or is not practical by ahost in the forwarding path. This condition may result whenan ESMTP gateway supports 8bit transport but is not able todowngrade the message to 7 bit as required for the next hop. X.6.4 Conversion with loss performedThis is a warning sent to the sender when message deliverywas successfully but when the delivery required a conversionin which some data was lost. This may also be a permananterror if the sender has indicated that conversion with lossis prohibited for the message. X.6.5 Conversion FailedA conversion was required but was unsuccessful. This may beuseful as a permanent or persistent temporary notification.
3.8 Security or Policy Status X.7.0 Other or undefined security statusSomething related to security caused the message to bereturned, and the problem cannot be well expressed with anyof the other provided detail codes. This status code mayalso be used when the condition cannot be further describedbecause of security policies in force. X.7.1 Delivery not authorized, message refusedThe sender is not authorized to send to the destination.This can be the result of per-host or per-recipientfiltering. This memo does not discuss the merits of anysuch filtering, but provides a mechanism to report such.This is useful only as a permanent error. X.7.2 Mailing list expansion prohibitedThe sender is not authorized to send a message to theintended mailing list. This is useful only as a permanenterror. X.7.3 Security conversion required but not possibleA conversion from one secure messaging protocol to anotherwas required for delivery and such conversion was notpossible. This is useful only as a permanent error. X.7.4 Security features not supportedA message contained security features such as secureauthentication which could not be supported on the deliveryprotocol. This is useful only as a permanent error. X.7.5 Cryptographic failureA transport system otherwise authorized to validate ordecrypt a message in transport was unable to do so becausenecessary information such as key was not available or suchinformation was invalid. X.7.6 Cryptographic algorithm not supportedA transport system otherwise authorized to validate ordecrypt a message was unable to do so because the necessaryalgorithm was not supported. X.7.7 Message integrity failureA transport system otherwise authorized to validate amessage was unable to do so because the message wascorrupted or altered. This may be useful as a permanent,transient persistent, or successful delivery code.4. References [SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC821, USC/Information Sciences Institute, August 1982. [DSN] Moore, K., and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC1894, University of Tennessee, Octel Network Services, January 1996.5. Security Considerations This document describes a status code system with increased precision. Use of these status codes may disclose additional information about how an internal mail system is implemented beyond that currently available.6. Acknowledgments The author wishes to offer special thanks to Harald Alvestrand, Marko
Kaittola, and Keith Moore for their extensive review and constructive suggestions.7. Author"s Address Gregory M. Vaudreuil Octel Network Services 17060 Dallas Parkway Suite 214 Dallas, TX 75248-1905 Voice/Fax: +1-214-733-2722 EMail: Greg.Vaudreuil@Octel.com8. Appendix - Collected Status Codes X.1.0 Other address status X.1.1 Bad destination mailbox address X.1.2 Bad destination system address X.1.3 Bad destination mailbox address syntax X.1.4 Destination mailbox address ambiguous X.1.5 Destination mailbox address valid X.1.6 Mailbox has moved X.1.7 Bad sender"s mailbox address syntax X.1.8 Bad sender"s system address X.2.0 Other or undefined mailbox status X.2.1 Mailbox disabled, not accepting messages X.2.2 Mailbox full X.2.3 Message length exceeds administrative limit. X.2.4 Mailing list expansion problem X.3.0 Other or undefined mail system status X.3.1 Mail system full X.3.2 System not accepting network messages X.3.3 System not capable of selected features X.3.4 Message too big for system X.4.0 Other or undefined network or routing status X.4.1 No answer from host X.4.2 Bad connection X.4.3 Routing server failure X.4.4 Unable to route X.4.5 Network congestion X.4.6 Routing loop detected X.4.7 Delivery time expired X.5.0 Other or undefined protocol status X.5.1 Invalid command X.5.2 Syntax error X.5.3 Too many recipients X.5.4 Invalid command arguments X.5.5 Wrong protocol version X.6.0 Other or undefined media error X.6.1 Media not supported X.6.2 Conversion required and prohibited X.6.3 Conversion required but not supported X.6.4 Conversion with loss performed X.6.5 Conversion failed X.7.0 Other or undefined security status X.7.1 Delivery not authorized, message refused X.7.2 Mailing list expansion prohibited X.7.3 Security conversion required but not possible X.7.4 Security features not supported X.7.5 Cryptographic failure X.7.6 Cryptographic algorithm not supported X.7.7 Message integrity failure