Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry (last updated 2008-09-05) Registries included below: - Class Sub-Codes - Subject Sub-Codes - Enumerated Status Codes Registry Name: Class Sub-Codes Reference: [RFC5248] Registration Procedures: Specification Required Registry: Code: 2.XXX.YYY Summary: Success Description: Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: 4.XXX.YYY Summary: Persistent Transient Failure Description: A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: 5.XXX.YYY Summary: Permanent Failure Description: 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. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Registry Name: Subject Sub-Codes Reference: [RFC5248] Registration Procedures: Specification Required Registry: Code: X.0.YYY Summary: Other or Undefined Status Description: There is no additional subject information available. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.1.YYY Summary: Addressing Status Description: The address status reports on the originator or destination address. It may include address syntax or validity. These errors can generally be corrected by the sender and retried. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.2.YYY Summary: Mailbox Status Description: Mailbox status indicates that something having to do with the mailbox has caused this DSN. Mailbox issues are assumed to be under the general control of the recipient. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.3.YYY Summary: Mail System Status Description: Mail system status indicates that something having to do with the destination system has caused this DSN. System issues are assumed to be under the general control of the destination system administrator. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.4.YYY Summary: Network and Routing Status Description: The networking or routing codes report status about the delivery system itself. These system components include any necessary infrastructure such as directory and routing services. Network issues are assumed to be under the control of the destination or intermediate system administrator. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.5.YYY Summary: Mail Delivery Protocol Status Description: The mail delivery protocol status codes report failures involving the message delivery protocol. These failures include the full range of problems resulting from implementation errors or an unreliable connection. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.6.YYY Summary: Message Content or Media Status Description: The message content or media status codes report failures involving the content of the message. These codes report failures due to translation, transcoding, or otherwise unsupported message media. Message content or media issues are under the control of both the sender and the receiver, both of which must support a common set of supported content-types. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Code: X.7.YYY Summary: Security or Policy Status Description: The security or policy status codes report failures involving policies such as per-recipient or per-host filtering and cryptographic operations. Security and policy status issues are assumed to be under the control of either or both the sender and recipient. Both the sender and recipient must permit the exchange of messages and arrange the exchange of necessary keys and certificates for cryptographic operations. Reference: [RFC3463] (Standards track) Submitter: G. Vaudreuil Change Controller: IESG Registry Name: Enumerated Status Codes Reference: [RFC5248] Registration Procedures: Specification Required Registry: Code: X.0.0 Sample Text: Other undefined Status Associated basic status code: Any Description: Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.0 Sample Text: Other address status Associated basic status code: Not given Description: Something about the address specified in the message caused this DSN. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.1 Sample Text: Bad destination mailbox address Associated basic status code: 451, 550 Description: The mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the "@" sign is invalid. This code is only useful for permanent failures. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.2 Sample Text: Bad destination system address Associated basic status code: Not given Description: The destination system specified in the address does not exist or is incapable of accepting mail. For Internet mail names, this means the address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.3 Sample Text: Bad destination mailbox address syntax Associated basic status code: 501 Description: The destination address was syntactically invalid. This can apply to any field in the address. This code is only useful for permanent failures. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.4 Sample Text: Destination mailbox address ambiguous Associated basic status code: Not given Description: The mailbox address as specified matches one or more recipients on the destination system. This may result if a heuristic address mapping algorithm is used to map the specified address to a local mailbox name. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.5 Sample Text: Destination address valid Associated basic status code: 250 Description: This mailbox address as specified was valid. This status code should be used for positive delivery reports. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.6 Sample Text: Destination mailbox has moved, No forwarding address Associated basic status code: Not given Description: The mailbox address provided was at one time valid, but mail is no longer being accepted for that address. This code is only useful for permanent failures. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.7 Sample Text: Bad sender's mailbox address syntax Associated basic status code: Not given Description: The sender's address was syntactically invalid. This can apply to any field in the address. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.8 Sample Text: Bad sender's system address Associated basic status code: 451, 501 Description: The sender's system specified in the address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the "@" is invalid for mail. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.1.9 Sample Text: Message relayed to non-compliant mailer Associated basic status code: Not given Description: The mailbox address specified was valid, but the message has been relayed to a system that does not speak this protocol; no further information can be provided. Reference: [RFC3886] (Standards Track) Submitter: E. Allman Change controller: IESG Code: X.2.0 Sample Text: Other or undefined mailbox status Associated basic status code: Not given Description: The mailbox exists, but something about the destination mailbox has caused the sending of this DSN. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.2.1 Sample Text: Mailbox disabled, not accepting messages Associated basic status code: Not given Description: The mailbox exists, but is not accepting messages. This may be a permanent error if the mailbox will never be re-enabled or a transient error if the mailbox is only temporarily disabled. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.2.2 Sample Text: Mailbox full Associated basic status code: 552 Description: The mailbox is full because the user has exceeded a per-mailbox administrative quota or physical capacity. The general semantics implies that the recipient can delete messages to make more space available. This code should be used as a persistent transient failure. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.2.3 Sample Text: Message length exceeds administrative limit Associated basic status code: 552 Description: A per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit. This code should be used as a permanent failure. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.2.4 Sample Text: Mailing list expansion problem Associated basic status code: 450, 452 Description: The mailbox is a mailing list address and the mailing list was unable to be expanded. This code may represent a permanent failure or a persistent transient failure. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.0 Sample Text: Other or undefined mail system status Associated basic status code: 221, 250, 421, 451, 550, 554 Description: The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.1 Sample Text: Mail system full Associated basic status code: 452 Description: Mail system storage has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages. This is useful only as a persistent transient error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.2 Sample Text: System not accepting network messages Associated basic status code: 453 Description: The host on which the mailbox is resident is not accepting messages. Examples of such conditions include an immanent shutdown, excessive load, or system maintenance. This is useful for both permanent and persistent transient errors. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.3 Sample Text: System not capable of selected features Associated basic status code: Not given Description: Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.4 Sample Text: Message too big for system Associated basic status code: 552, 554 Description: The message is larger than per-message size limit. This limit may either be for physical or administrative reasons. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.3.5 Sample Text: System incorrectly configured Associated basic status code: Not given Description: The system is not configured in a manner that will permit it to accept this message. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.0 Sample Text: Other or undefined network or routing status Associated basic status code: Not given Description: Something went wrong with the networking, but it is not clear what the problem is, or the problem cannot be well expressed with any of the other provided detail codes. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.1 Sample Text: No answer from host Associated basic status code: 451 Description: The outbound connection attempt was not answered, because either the remote system was busy, or was unable to take a call. This is useful only as a persistent transient error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.2 Sample Text: Bad connection Associated basic status code: 421 Description: The outbound connection was established, but was unable to complete the message transaction, either because of time-out, or inadequate connection quality. This is useful only as a persistent transient error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.3 Sample Text: Directory server failure Associated basic status code: 451, 550 Description: The network system was unable to forward the message, because a directory server was unavailable. This is useful only as a persistent transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.4 Sample Text: Unable to route Associated basic status code: Not given Description: The mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server. This is useful for both 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 to route error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.5 Sample Text: Mail system congestion Associated basic status code: 451 Description: The mail system was unable to deliver the message because the mail system was congested. This is useful only as a persistent transient error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.6 Sample Text: Routing loop detected Associated basic status code: Not given Description: A routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user- forwarding loop. This is useful only as a persistent transient error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.4.7 Sample Text: Delivery time expired Associated basic status code: Not given Description: The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live value specified by the sender of the message was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.0 Sample Text: Other or undefined protocol status Associated basic status code: 220, 250, 251, 252, 253, 451, 452, 454, 458, 459, 501, 502, 503, 554 Description: Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.1 Sample Text: Invalid command Associated basic status code: 430, 500, 501, 503, 530, 550, 554, 555 Description: A mail transaction protocol command was issued which was either out of sequence or unsupported. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.2 Sample Text: Syntax error Associated basic status code: 500, 501, 502, 550, 555 Description: A mail transaction protocol command was issued which could not be interpreted, either because the syntax was wrong or the command is unrecognized. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.3 Sample Text: Too many recipients Associated basic status code: 451 Description: More recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the segmentation of the message into two, the remainder of the recipients to be delivered on a subsequent delivery attempt. It is included in this list in the event that such segmentation is not possible. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.4 Sample Text: Invalid command arguments Associated basic status code: 451, 501, 502, 503, 504, 550, 555 Description: A valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.5 Sample Text: Wrong protocol version Associated basic status code: Not given Description: A protocol version mis-match existed which could not be automatically resolved by the communicating parties. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.5.6 Sample Text: Authentication Exchange line is too long Associated basic status code: 500 Description: This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client sending a [BASE64] response which is longer than the maximum buffer size available for the currently selected SASL mechanism. This is useful for both permanent and persistent transient errors. Reference: [RFC4954] (Standards Track) Submitter: R. Siemborski, A. Melnikov Change controller: IESG Code: X.6.0 Sample Text: Other or undefined media error Associated basic status code: Not given Description: Something about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.1 Sample Text: Media not supported Associated basic status code: Not given Description: The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.2 Sample Text: Conversion required and prohibited Associated basic status code: Not given Description: The content of the message must be converted before it can be delivered and such conversion is not permitted. Such prohibitions may be the expression of the sender in the message itself or the policy of the sending host. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.3 Sample Text: Conversion required but not supported Associated basic status code: 554 Description: The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path. This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.4 Sample Text: Conversion with loss performed Associated basic status code: 250 Description: This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.5 Sample Text: Conversion Failed Associated basic status code: Not given Description: A conversion was required but was unsuccessful. This may be useful as a permanent or persistent temporary notification. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.6.6 Sample Text: Message content not available Associated basic status code: 554 Description: The message content could not be fetched from a remote system. This may be useful as a permanent or persistent temporary notification. Reference: [RFC4468] (Standards Track) Submitter: C. Newman Change controller: IESG Code: X.6.7 Sample Text: The ALT-ADDRESS is required but not specified Associated basic status code: 553, 550 Description: This indicates the reception of a MAIL or RCPT command that required an ALT-ADDRESS parameter but such parameter was not present. Defined: [RFC5336] (Experimental track) Submitter: Jiankang YAO Change controller: IESG Code: X.6.8 Sample Text: UTF-8 string reply is required, but not permitted by the client Associated basic status code: 553, 550 Description: This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the client. Defined: [RFC5336] (Experimental track) Submitter: Jiankang YAO Change controller: IESG Code: X.6.9 Sample Text: UTF8SMTP downgrade failed Associated basic status code: 550 Description: This indicates that transaction failed after the final "." of the DATA command. Defined: [RFC5336] (Experimental track) Submitter: Jiankang YAO Change controller: IESG Code: X.6.10 Sample Text: UTF-8 string reply is required, but not permitted by the client Associated basic status code: 252 Description: This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the client. Defined: [RFC5336] (Experimental track) Submitter: Jiankang YAO Change controller: IESG Code: X.7.0 Sample Text: Other or undefined security status Associated basic status code: 220, 235, 450, 454, 500, 501, 503, 504, 530, 535, 550 Description: Something related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes. This status code may also be used when the condition cannot be further described because of security policies in force. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.1 Sample Text: Delivery not authorized, message refused Associated basic status code: 451, 454, 502, 503, 533, 550, 551 Description: The sender is not authorized to send to the destination. This can be the result of per-host or per-recipient filtering. This memo does not discuss the merits of any such filtering, but provides a mechanism to report such. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.2 Sample Text: Mailing list expansion prohibited Associated basic status code: 550 Description: The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.3 Sample Text: Security conversion required but not possible Associated basic status code: Not given Description: A conversion from one secure messaging protocol to another was required for delivery and such conversion was not possible. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.4 Sample Text: Security features not supported Associated basic status code: 504 Description: A message contained security features such as secure authentication that could not be supported on the delivery protocol. This is useful only as a permanent error. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.5 Sample Text: Cryptographic failure Associated basic status code: Not given Description: A transport system otherwise authorized to validate or decrypt a message in transport was unable to do so because necessary information such as key was not available or such information was invalid. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.6 Sample Text: Cryptographic algorithm not supported Associated basic status code: Not given Description: A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.7 Sample Text: Message integrity failure Associated basic status code: Not given Description: A transport system otherwise authorized to validate a message was unable to do so because the message was corrupted or altered. This may be useful as a permanent, transient persistent, or successful delivery code. Reference: [RFC3463] (Standards Track) Submitter: G. Vaudreuil Change controller: IESG Code: X.7.8 Sample Text: Authentication credentials invalid Associated basic status code: 535, 554 Description: This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box). Reference: [RFC4954] (Standards Track) Submitter: R. Siemborski, A. Melnikov Change controller: IESG Code: X.7.9 Sample Text: Authentication mechanism is too weak Associated basic status code: 534 Description: This response to the AUTH command indicates that the selected authentication mechanism is weaker than server policy permits for that user. The client SHOULD retry with a new authentication mechanism. Reference: [RFC4954] (Standards Track) Submitter: R. Siemborski, A. Melnikov Change controller: IESG Code: X.7.10 Sample Text: Encryption Needed Associated basic status code: 523 Description: This indicates that external strong privacy layer is needed in order to use the requested authentication mechanism. This is primarily intended for use with clear text authentication mechanisms. A client which receives this may activate a security layer such as TLS prior to authenticating, or attempt to use a stronger mechanism. Reference: [RFC5248] (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG Code: X.7.11 Sample Text: Encryption required for requested authentication mechanism Associated basic status code: 524, 538 Description: This response to the AUTH command indicates that the selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. Note that this response code is documented here for historical purposes only. Modern implementations SHOULD NOT advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed. Reference: [RFC4954] (Standards Track) Submitter: R. Siemborski, A. Melnikov Change controller: IESG Code: X.7.12 Sample Text: A password transition is needed Associated basic status code: 422, 432 Description: This response to the AUTH command indicates that the user needs to transition to the selected authentication mechanism. This is typically done by authenticating once using the [PLAIN] authentication mechanism. The selected mechanism SHOULD then work for authentications in subsequent sessions. Reference: [RFC4954] (Standards Track) Submitter: R. Siemborski, A. Melnikov Change controller: IESG Code: X.7.13 Sample Text: User Account Disabled Associated basic status code: 525 Description: Sometimes a system administrator will have to disable a user's account (e.g., due to lack of payment, abuse, evidence of a break-in attempt, etc). This error code occurs after a successful authentication to a disabled account. This informs the client that the failure is permanent until the user contacts their system administrator to get the account re-enabled. It differs from a generic authentication failure where the client's best option is to present the passphrase entry dialog in case the user simply mistyped their passphrase. Reference: [RFC5248] (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG Code: X.7.14 Sample Text: Trust relationship required Associated basic status code: 535, 554 Description: The submission server requires a configured trust relationship with a third-party server in order to access the message content. This value replaces the prior use of X.7.8 for this error condition. thereby updating [RFC4468]. Reference: [RFC5248] (Best current practice) Submitter: T. Hansen, J. Klensin Change controller: IESG References ---------- [RFC3463] G. Vaudreuil, "Enhanced Mail System Status Codes", RFC 3463, January 2003. [RFC3886] E. Allman, "An Extensible Message Format for Message Tracking Responses", RFC 3886, September 2004. [RFC4468] C. Newman, "Message Submission BURL Extension", RFC 4468, May 2006. [RFC4954] R. Siemborski, Ed., A. Melnikov, Ed., "SMTP Service Extension for Authentication", RFC 4954, July 2007. [RFC5248] T. Hansen, J. Klensin, "A Registry for SMTP Enhanced Mail System Status Codes", RFC 5248, June 2008. [RFC5336] J. Yao, Ed., W. Mao, Ed., "SMTP extension for internationalized email address", RFC 5336, September 2008. (created 2008-05-29)