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SMTP While EMAILCORE working group was working on update to SMTP specification, it became clear that existing entries in SMTP related registries are missing information or contain stale information and need to be updated. This document updates such entries.

While EMAILCORE working group was working on update to SMTP specification , it became clear that existing entries in SMTP related registries are missing information or contain stale information and need to be updated. This document updates such entries, assuming IANA actions specified in were already performed. This document doesn't do any restructuring to existing SMTP related registries.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Updates to the "Simple Mail Transfer Protocol (SMTP) Service Extensions" registry are described as an ordered list of steps that should be performed sequentually. Each step is described by a subsection of this section.

IANA is requested to change values of the "References" field for "EXPN" and "HELP" extensions to . The "References" field values for "SEND", "SOML", "SAML" and "TURN" remain unchanged, as they were obsoleted and there are no newer reference than RFC 821.

IANA is requested to update the following values in this field: The value corresponding to the DELIVERBY extension should be changed to "A comma separated list of options (optional)", as per . The value corresponding to the FUTURERELEASE extension should be changed to "max-future-release-interval and max-future-release-date-time", as per . The value corresponding to the LIMITS extension should be changed to "List of limits", as per .

IANA is requested to update values of the "Message submission Use and Values" field corresponding to "HELP", "BURL", "FUTURERELEASE", "SMTPUTF8", "MT-PRIORITY", "RRVS", "REQUIRETLS" and "LIMITS" extensions to "MAY".

IANA is requested to update "Additional verbs" and "MAIL/RCPT Parameter Values" fields as per the table specified below. Information for relevant fields was extracted from existing RFCs describing corresponding SMTP Extensions. EHLO KeywordAdditional verbsMAIL/RCPT Parameter Values SENDSENDnone SOMLSOMLnone SAMLSAMLnone EXPNEXPNnone HELPHELPnone TURNTURNnone 8BITMIMEnoneBODY parameter to MAIL command SIZEnoneSIZE parameter to MAIL command CHUNKINGBDATnone BINARYMIMEnoneBODY parameter to MAIL command (an extra choice was added) CHECKPOINTnoneTRANSID parameter to the MAIL command DELIVERBYnoneBY parameter to MAIL command PIPELININGnonenone DSNnoneNOTIFY and ORCPT parameters to RCPT command. RET and ENVID parameters to MAIL command ETRNETRNnone ENHANCEDSTATUSCODESnonenone STARTTLSSTARTTLSnone NO-SOLICITINGnoneSOLICIT parameter to MAIL command MTRKnoneMTRK parameters to MAIL command. Also see DSN extension parameters SUBMITTERnoneSUBMITTER parameters to MAIL command ATRNATRNnone AUTHAUTHAUTH parameter to MAIL command BURLBURLnone FUTURERELEASEnoneHOLDFOR and HOLDUNTIL parameters to MAIL command. UTF8SMTPnoneOne optional parameter, ALT-ADDRESS, is added to the MAIL and RCPT commands CONPERMnoneCONPERM parameter to MAIL command CONNEGnoneCONNEG parameter to RCPT command SMTPUTF8noneSMTPUTF8 parameter to the MAIL command MT-PRIORITYnoneMT-PRIORITY parameter to MAIL command RRVSnoneRRVS parameter to RCPT command REQUIRETLSnoneREQUIRETLS parameter to MAIL command LIMITSnonenone IANA is further requested to update "Length Added" fields as per the table specified below. Information for this field was either extracted or calculated from information in existing RFCs describing corresponding SMTP Extensions. EHLO KeywordLength AddedComment SIZE26https://datatracker.ietf.org/doc/html/rfc1870#section-3 CHUNKING0https://datatracker.ietf.org/doc/html/rfc3030#section-2 BINARYMIME16https://datatracker.ietf.org/doc/html/rfc3030#section-3 CHECKPOINT88https://datatracker.ietf.org/doc/html/rfc1845#section-2 DELIVERBY17https://datatracker.ietf.org/doc/html/rfc2852#section-2 MTRK40https://datatracker.ietf.org/doc/html/rfc3885#section-2 AUTH500https://datatracker.ietf.org/doc/html/rfc4954#section-3 FUTURERELEASE34https://datatracker.ietf.org/doc/html/rfc4865#section-3 UTF8SMTP460https://datatracker.ietf.org/doc/html/rfc5336#section-3.1 SMTPUTF810https://datatracker.ietf.org/doc/html/rfc6531#section-3.1 MT-PRIORITY15https://datatracker.ietf.org/doc/html/rfc6710#section-3 RRVS33https://datatracker.ietf.org/doc/html/rfc7293#section-3.1 REQUIRETLS11https://www.rfc-editor.org/rfc/rfc8689.html#name-iana-considerations TBD: Calculate "Length Added" when not specified in existing RFCs. Also possibly merge this with the previous table? IANA is further requested to update "Message Submission Use and Values" fields as per the table specified below. Most of these extensions were published since RFC 6409 was published, so they are not explicitly listed in it. In regards to HELP extension, there is no particular reason to prohibit it on Submission. EHLO KeywordMessage Submission Use and Values HELPMAY BURLMAY FUTURERELEASEMAY SMTPUTF8MAY MT-PRIORITYMAY RRVSMAY REQUIRETLSMAY LIMITSMAY

This document only specifies IANA related actions, thus it doesn't directly affect security on SMTP or its extensions. However better documentation of SMTP extensions in IANA registries can, hopefully, lead to better and more secure implementations.

This document is a specification of the basic protocol for Internet electronic mail transport. It (including text carried forward from RFC 5321) consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. The document also provides information about use of SMTP for other than strict mail transport and delivery. This document replaces RFC 5321, the earlier version with the same title, and supersedes RFCs 1846, 7504, and 7505, incorporating all the relevant information in them.

Thank you to John Klensin and Tero Kivinen for suggestions, comments and corrections on this document. Tero Kivinen provided initial version of the table with Length Added values for various SMTP extensions.