Internet Assigned Numbers Authority MAIL Parameters Last Updated 2024-02-08 Available Formats [IMG] XML [IMG] HTML [IMG] Plain text Registries included below • SMTP Service Extensions • SMTP Service Extension Parameters • Mail Transmission Types • VIA link types • WITH protocol types • Additional-registered-clauses • SMTP PRIORITY extension Priority Assignment Policy • Registered-states • Multicast Email SMTP Extensions • SMTP Server Limits SMTP Service Extensions Registration Procedure(s) Standards Action or IESG-Approved Experimental RFC Reference [RFC5321, Section 2.2] Note The Simple Mail Transfer Protocol [RFC5321] specifies a set of commands or services and a general procedure for extending that set. The table below lists SMTP service extensions. Both message submission [RFC6409] and mail transfer [RFC5321] may use these extensions unless otherwise specified. As specified in [RFC5321] Section 2.2.2, no entry may be made in this registry that starts in an "X". Entries may be made only for service extensions (and associated keywords, parameters, or verbs) that are defined in Standards-Track or Experimental RFCs specifically approved by the IESG for this purpose. The server response to a client EHLO command includes a keyword for each service extension the server implements. Some of these keywords have parameters, which are listed in the "SMTP Service Extension Parameters" registry. Note The SMTP Extension names/keywords registered in this section are for general use. For those specific to Multicast email, see the registry for that protocol below. Available Formats [IMG] CSV EHLO Keyword Description Reference Note SEND Send to terminal [RFC821] The description of SEND was updated by [RFC1123] and then its actual use was deprecated in [RFC2821] SOML Send as mail or to [RFC821] The description of SOML was updated by [RFC1123] and then its actual use was terminal deprecated in [RFC2821] SAML Send as mail and to [RFC821] The description of SAML was updated by [RFC1123] and then its actual use was terminal deprecated in [RFC2821] EXPN Expand the mailing list [RFC821] Command description updated by [RFC5321] HELP Supply helpful [RFC821] Command description updated by [RFC5321] information TURN Interchange the client [RFC821] The actual use of TURN was deprecated in [RFC2821] and server roles SMTP and Submit 8BITMIME transport of 8bit MIME [RFC6152] content SIZE Message size [RFC1870] For additional declarations or limits of this general type, see the LIMITS extension declaration below and the SMTP Server Limits registry. VERB Verbose [Eric Allman] ONEX One message transaction [Eric Allman] only CHUNKING Chunking [RFC3030] BINARYMIME Binary MIME [RFC3030] CHECKPOINT Checkpoint/Restart [RFC1845] DELIVERBY Deliver By [RFC2852] PIPELINING Command Pipelining [RFC2920] DSN Delivery Status [RFC3461] Notification ETRN Extended Turn [RFC1985] SMTP [RFC5321] only. Not for use on Submit port 587. ENHANCEDSTATUSCODES Enhanced Status Codes [RFC2034] STARTTLS Start TLS [RFC3207] NO-SOLICITING Notification of no [RFC3865] soliciting MTRK Message Tracking [RFC3885] SUBMITTER SMTP Responsible [RFC4405] Deprecated by Submitter [https://datatracker.ietf.org/doc/status-change-change-sender-id-to-historic]. ATRN Authenticated TURN [RFC2645] SMTP [RFC5321] only. Not for use on Submit port 587. AUTH Authentication [RFC4954] BURL Remote Content [RFC4468] Submit [RFC6409] only. Not for use with SMTP on port 25. FUTURERELEASE Future Message Release [RFC4865] UTF8SMTP Internationalized email [RFC5336] Experimental; deprecated in [RFC6531]. address CONPERM Content Conversion [RFC4141] Permission CONNEG Content Conversion [RFC4141] Negotiation SMTPUTF8 Internationalized email [RFC6531] address MT-PRIORITY Priority Message [RFC6710] Handling RRVS Require Recipient Valid [RFC7293] Since REQUIRETLS Require TLS [RFC8689] LIMITS Server limits [RFC9422] See "SMTP Server Limits" registry. SMTP Service Extension Parameters Registration Procedure(s) Standards Action or IESG-Approved Experimental RFC Reference [RFC5321, Section 2.2] Note Some of the keywords in the "SMTP Service Extensions" registry have parameters. These are listed here (those keywords not listed here have no parameters). Available Formats [IMG] CSV EHLO Keyword Parameters Reference SIZE number [RFC1870] NO-SOLICITING Soliciting-keywords [RFC3865] AUTH SASL mechanism names [RFC4954] BURL Permitted URL prefixes [RFC4468] MT-PRIORITY Name of the Priority Assignment Policy (optional) [RFC6710] Mail Transmission Types Registration Procedure(s) Standards Action or IESG-Approved Experimental RFC Reference [RFC821][RFC822][RFC5321] Note The Simple Mail Transfer Protocol [RFC821][RFC5321] and the Standard for the Format of ARPA Internet Text Messages [RFC822] specify that a set of "Received" lines will be prepended to the headers of electronic mail messages as they are transported through the Internet. These received line may optionally include either or both a "via" phrase and/or a "with" phrase. The legal values for the phrases are listed here. The via phrase is intended to indicate the link or physical medium over which the message was transferred. The with phrase is intended to indicate the protocol or logical process that was used to transfer the message. VIA link types Available Formats [IMG] CSV VIA link types Description Reference UUCP Unix-to-Unix Copy Program TCP TCP [RFC5321] WITH protocol types Available Formats [IMG] CSV WITH protocol types Description Reference SMTP Simple Mail Transfer Protocol [RFC5321] ESMTP SMTP with Service Extensions [RFC5321] ESMTPA ESMTP with AUTH [RFC3848] ESMTPS ESMTP with STARTTLS [RFC3848] ESMTPSA ESMTP with both STARTTLS and AUTH [RFC3848] LMTP Local Mail Transfer Protocol [RFC3848] LMTPA LMTP with AUTH [RFC3848] LMTPS LMTP with STARTTLS [RFC3848] LMTPSA LMTP with both STARTTLS and AUTH [RFC3848] MMS Multimedia Messaging Service [RFC4356] UTF8SMTP ESMTP with SMTPUTF8 [RFC6531] UTF8SMTPA ESMTP with SMTPUTF8 and AUTH [RFC4954][RFC6531] UTF8SMTPS ESMTP with SMTPUTF8 and STARTTLS [RFC3207][RFC6531] UTF8SMTPSA ESMTP with SMTPUTF8 and both STARTTLS and AUTH [RFC3207][RFC4954][RFC6531] UTF8LMTP LMTP with SMTPUTF8 [RFC6531] UTF8LMTPA LMTP with SMTPUTF8 and AUTH [RFC4954][RFC6531] UTF8LMTPS LMTP with SMTPUTF8 and STARTTLS [RFC3207][RFC6531] UTF8LMTPSA LMTP with SMTPUTF8 and both STARTTLS and AUTH [RFC3207][RFC4954][RFC6531] Additional-registered-clauses Registration Procedure(s) Standards Action or IESG-Approved Experimental RFC Reference [RFC5321] Available Formats [IMG] CSV Clause Name Description Syntax Summary Reference PRIORITY Records the value of the MT-PRIORITY parameter specified in the See Section 7 of RFC [RFC6710] [RFC6710] MAIL FROM command state Indicates entry into a special queue state state [RFC6729] tls-cipher = tls-cipher-name / tls-cipher-hex tls Indicates the TLS cipher used tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") [RFC8314] ; as registered in the IANA TLS Cipher Suite Registry tls-cipher-hex = "0x" 4HEXDIG Indicates the Diffie-Hellman group used with the TLS cipher (if dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-") group applicable) ; as registered in the IANA TLS Supported Groups [RFC8314] Registry SMTP PRIORITY extension Priority Assignment Policy Registration Procedure(s) Specification Required Expert(s) Unassigned Reference [RFC6710] Available Formats [IMG] CSV Policy Name Comment Reference MIXER Default policy [RFC6710, Appendix B] STANAG4406 [RFC6710, Appendix A] NSEP [RFC6710, Appendix C] Registered-states Registration Procedure(s) First Come First Served Reference [RFC6729] Available Formats [IMG] CSV Name Description Reference Use auth Held for message authentication [RFC6729] current content Held for message content analysis [RFC6729] current convert Held for message content conversion [RFC6729] current moderation Held for list moderation [RFC6729] current normal Message is not being held other than to accommodate typical relaying handling [RFC6729] current other Held for causes not covered by other registered state keywords [RFC6729] current outbound Message placed in outbound queue [RFC6729] current quarantine Held for operator action due to content analysis or local policy [RFC6729] current timed Held to accommodate a specific requested delivery window [RFC6729] current Multicast Email SMTP Extensions Registration Procedure(s) Specification Required Expert(s) Alexey Melnikov (primary), RFC Independent Submissions Editor (secondary) Reference [RFC8494] Note Extension names/keywords and status used with Multicast Email; see SMTP Service Extensions above for the list for more general use. Available Formats [IMG] CSV Name Status Multicast-Mail-Specific Non-Multicast-Specific Note References References SIZE Required [RFC1870] 8BITMIME Required [RFC6152] DSN Required [RFC3461] MT-PRIORITY Required [RFC6710] DELIVERBY Required [RFC2852] BINARYMIME Required [RFC3030] SMTP CHUNKING MUST be supported on the receiving SMTP side of an SMTP-to-MULE CHUNKING Special [RFC3030] gateway and MAY be used on the sending side of a MULE-to-SMTP gateway. A MULE relay doesn't need to do anything special for this extension. ENHANCEDSTATUSCODES Special [RFC2034] The ENHANCEDSTATUSCODES extension is supported by including relevant status codes in DSN [RFC3461] reports. RRVS Supported [RFC7293] SUBMITTER Supported [RFC4405] Deprecated by [https://datatracker.ietf.org/doc/status-change-change-sender-id-to-historic]. PIPELINING N/A [RFC2920] STARTTLS N/A [RFC3207] AUTH Special [RFC4954] The AUTH parameter to the MAIL FROM command is "Supported", but the rest of the AUTH extension is not applicable to MULE. BURL N/A [RFC4468] NO-SOLICITING N/A [RFC3865] CHECKPOINT Disallowed [RFC1845] CONNEG Disallowed [RFC4141] SMTP Server Limits Registration Procedure(s) Specification Required Expert(s) Unassigned Reference [RFC9422] Available Formats [IMG] CSV Name Value syntax Description Restrictions Security Considerations Reference MAILMAX specifies the maximum number of transactions MAILMAX %x31-39 0*5DIGIT ; "0" not (MAIL FROM commands) the server will accept in a single None See [RFC9422, Section 6] [RFC9422] allowed, six-digit maximum session. The count includes all MAIL FROM commands, regardless of whether they succeed or fail. RCPTMAX specifies the maximum number of RCPT TO commands the server will accept in a single RCPTMAX %x31-39 0*5DIGIT ; "0" not transaction. It is not a limit on the actual number of None See [RFC9422, Section 6] [RFC9422] allowed, six-digit maximum recipients the message ends up being sent to; a single RCPT TO command may produce multiple recipients or, in the event of an error, none. RCPTDOMAINMAX specifies the maximum number of different domains that can appear in a recipient (RCPT TO) RCPTDOMAINMAX %x31-39 0*5DIGIT ; "0" not address within a single session. This limit is imposed None See [RFC9422, Section 6] [RFC9422] allowed, six-digit maximum by some servers that bind to a specific internal delivery mechanism on receipt of the first RCPT TO command. Licensing Terms