Internet Assigned Numbers Authority SMTP-Related Registries Last Updated 2025-05-02 Note This registry group was formerly known as "MAIL Parameters." Associated Registries [https://www.iana.org/assignments/message-headers] Available Formats [IMG] XML [IMG] HTML [IMG] Plain text Registries Included Below • SMTP Service Extensions • SMTP Service Extension Parameters • Mail Transmission Types for the "Received:" Header Field • VIA Link Types • WITH Protocol Types • Additional Registered 'Received:' Clauses • Address Literal Tags • SMTP PRIORITY Extension Priority Assignment Policy • Registered-states • Multicast Email SMTP Extensions • SMTP Server Limits • Mail Header Confidentiality Policies SMTP Service Extensions Registration Procedure(s) Either Model 1 (IETF) or Model 2 (FCFS) as described in Sections 8.1.1.1 of [RFC-ietf-emailcore-rfc5321bis-43]. Reference [RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.1, 2.2.2] Note The Simple Mail Transfer Protocol [RFC-ietf-emailcore-rfc5321bis-43] 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 [RFC-ietf-emailcore-rfc5321bis-43] 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 MAIL/RCPT Message EHLO Keyword Description Reference EHLO Parameters Additional Parameter RegMethod Submission Length Note Contact Change Verbs Values Use and Added Controller Values The description of SEND was SEND Send to terminal [RFC821] None [RFC821] [RFC821] IETF MUST NOT updated by [RFC1123] and then its IETF IETF actual use was deprecated in [RFC2821] The description of SOML was SOML Send as mail or to terminal [RFC821] None [RFC821] [RFC821] IETF MUST NOT updated by [RFC1123] and then its IETF IETF actual use was deprecated in [RFC2821] The description of SAML was SAML Send as mail and to terminal [RFC821] None [RFC821] [RFC821] IETF MUST NOT updated by [RFC1123] and then its IETF IETF actual use was deprecated in [RFC2821] Implementation support for VRFY is required in servers, but its VRFY VRFY command as specified in [RFC-ietf-emailcore-rfc5321bis-43] None None None IETF MUST NOT 0 listing in the EHLO response is IETF IETF [RFC-ietf-emailcore-rfc5321bis-43] optional. See Section 3.5.2 in [RFC-ietf-emailcore-rfc5321bis-43] for details on this subject. EXPN Expand the mailing list [RFC821] None [RFC821] [RFC821] IETF MUST NOT Command description updated by IETF IETF [RFC-ietf-emailcore-rfc5321bis-43] HELP Supply helpful information [RFC821] None [RFC821] [RFC821] IETF MUST NOT Command description updated by IETF IETF [RFC-ietf-emailcore-rfc5321bis-43] TURN Interchange the client and server [RFC821] None [RFC821] [RFC821] IETF MUST NOT The actual use of TURN was IETF IETF roles deprecated in [RFC2821] 8BITMIME SMTP and Submit transport of 8bit [RFC6152] None [RFC6152] [RFC6152] IETF SHOULD IETF IETF MIME content For additional declarations or SIZE Message size declaration [RFC1870] number [RFC1870] [RFC1870] IETF MAY limits of this general type, see IETF IETF the LIMITS extension below and the SMTP Server Limits registry. VERB Verbose [Eric Allman] Not supplied Not Not Legacy Not Not Eric Eric supplied supplied supplied supplied Allman Allman ONEX One message transaction only [Eric Allman] Not supplied Not Not Legacy Not Not Eric Eric supplied supplied supplied supplied Allman Allman CHUNKING Chunking [RFC3030] None [RFC3030] [RFC3030] IETF MAY IETF IETF BINARYMIME Binary MIME [RFC3030] None [RFC3030] [RFC3030] IETF MAY IETF IETF CHECKPOINT Checkpoint/Restart [RFC1845] None [RFC1845] [RFC1845] IETF MAY IETF IETF DELIVERBY Deliver By [RFC2852] None [RFC2852] [RFC2852] IETF MAY IETF IETF PIPELINING Command Pipelining [RFC2920] None [RFC2920] [RFC2920] IETF SHOULD IETF IETF DSN Delivery Status Notification [RFC3461] None [RFC3461] [RFC3461] IETF SHOULD IETF IETF SMTP ETRN Extended Turn [RFC1985] None [RFC1985] [RFC1985] IETF MUST NOT [RFC-ietf-emailcore-rfc5321bis-43] IETF IETF only. Not for use on Submit port 587. ENHANCEDSTATUSCODES Enhanced Status Codes [RFC2034] None [RFC2034] [RFC2034] IETF SHOULD IETF IETF STARTTLS Start TLS [RFC3207] None [RFC3207] [RFC3207] IETF MAY IETF IETF NO-SOLICITING Notification of no soliciting [RFC3865] Soliciting-keywords [RFC3865] [RFC3865] IETF MAY IETF IETF MTRK Message Tracking [RFC3885] [RFC3885] [RFC3885] IETF MAY IETF IETF Deprecated by [Moving RFC 4405, SUBMITTER SMTP Responsible Submitter [RFC4405] None [RFC4405] [RFC4405] IETF MUST NOT RFC 4406, RFC 4407 (Sender-ID) to IETF IETF Historic]. SMTP ATRN Authenticated TURN [RFC2645] None [RFC2645] [RFC2645] IETF MUST NOT [RFC-ietf-emailcore-rfc5321bis-43] IETF IETF only. Not for use on Submit port 587. AUTH Authentication [RFC4954] SASL mechanism [RFC4954] [RFC4954] IETF MUST IETF IETF names BURL Remote Content [RFC4468] Permitted URL [RFC4468] [RFC4468] IETF MUST NOT Submit [RFC6409] only. Not for use IETF IETF prefixes with SMTP on port 25. FUTURERELEASE Future Message Release [RFC4865] None [RFC4865] [RFC4865] IETF MUST NOT IETF IETF UTF8SMTP Internationalized email address [RFC5336] None [RFC5336] [RFC5336] IETF MUST NOT Experimental; deprecated in IETF IETF [RFC6531]. CONPERM Content Conversion Permission [RFC4141] None [RFC4141] [RFC4141] IETF MAY IETF IETF CONNEG Content Conversion Negotiation [RFC4141] None [RFC4141] [RFC4141] IETF MAY IETF IETF SMTPUTF8 Internationalized email address [RFC6531] None [RFC6531] [RFC6531] IETF MUST NOT IETF IETF Name of the MT-PRIORITY Priority Message Handling [RFC6710] Priority Assignment [RFC6710] [RFC6710] IETF MUST NOT IETF IETF Policy (optional) RRVS Require Recipient Valid Since [RFC7293] None [RFC7293] [RFC7293] IETF MUST NOT IETF IETF REQUIRETLS Require TLS [RFC8689] None [RFC8689] [RFC8689] IETF MUST NOT IETF IETF LIMITS Server limits [RFC9422] None [RFC9422] [RFC9422] IETF MUST NOT See "SMTP Server Limits" registry. IETF IETF SMTP Service Extension Parameters Registration Procedure(s) Registry closed Reference [RFC-ietf-emailcore-rfc5321bis-43] Note These entries have been incorporated into the SMTP Service Extension registry's "EHLO Parameters" field. Mail Transmission Types for the "Received:" Header Field Reference [RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.3, 8.1.4][RFC-ietf-emailcore-rfc5322bis-12] Note The Simple Mail Transfer Protocol [RFC-ietf-emailcore-rfc5321bis-43] and the Standard for Internet Message Formats [RFC-ietf-emailcore-rfc5322bis-12] 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 Registration Procedure(s) RFC Required Reference [RFC-ietf-emailcore-rfc5321bis-43] Available Formats [IMG] CSV VIA link types Description Reference UUCP Unix-to-Unix Copy Program TCP TCP [RFC-ietf-emailcore-rfc5321bis-43] WITH Protocol Types Registration Procedure(s) RFC Required Reference [RFC-ietf-emailcore-rfc5321bis-43] Available Formats [IMG] CSV WITH protocol types Description Reference SMTP Simple Mail Transfer Protocol [RFC-ietf-emailcore-rfc5321bis-43] ESMTP SMTP with Service Extensions [RFC-ietf-emailcore-rfc5321bis-43] 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 'Received:' Clauses Registration Procedure(s) IETF Review Reference [RFC-ietf-emailcore-rfc5321bis-43] Available Formats [IMG] CSV Clause Name Service Extension Description Syntax Summary Reference PRIORITY Records the value of the MT-PRIORITY parameter specified in See Section 7 of RFC [RFC6710] [RFC6710] the MAIL FROM command state Indicates entry into a special queue state state [RFC6729] tls-cipher = tls-cipher-name / tls-cipher-hex tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") tls Indicates the TLS cipher used ; as registered in the IANA TLS Cipher Suite [RFC8314] Registry tls-cipher-hex = "0x" 4HEXDIG Indicates the Diffie-Hellman group used with the TLS cipher dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-") group (if applicable) ; as registered in the IANA TLS Supported Groups [RFC8314] Registry Address Literal Tags Registration Procedure(s) Standards Action Reference [RFC-ietf-emailcore-rfc5321bis-43] Note Sometimes a host is not known to the domain name system and communication (and, in particular, communication to report and repair the error) is blocked. To bypass this barrier a special literal form of the address is allowed as an alternative to a domain name. For IPv4 addresses, this form uses four small decimal integers separated by dots and enclosed by brackets such as [123.255.37.2], which indicates an (IPv4) Internet Address in sequence-of-octets form. For IPv6 and other forms of addressing that might eventually be standardized, the form consists of a standardized "tag" that identifies the address syntax, a colon, and the address itself, in a format specified as part of the IPv6 standards [17]. Available Formats [IMG] CSV Tag Reference IPv6 [RFC-ietf-emailcore-rfc5321bis-43] 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 References Non-Multicast-Specific Note 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 CHUNKING Special [RFC3030] side of an SMTP-to-MULE 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. The ENHANCEDSTATUSCODES extension is supported by ENHANCEDSTATUSCODES Special [RFC2034] including relevant status codes in DSN [RFC3461] reports. RRVS Supported [RFC7293] SUBMITTER Supported [RFC4405] Deprecated by [Moving RFC 4405, RFC 4406, RFC 4407 (Sender-ID) to Historic]. PIPELINING N/A [RFC2920] STARTTLS N/A [RFC3207] The AUTH parameter to the MAIL FROM command is AUTH Special [RFC4954] "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) John Klensin, Murray Kucherawy (backup) 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. Mail Header Confidentiality Policies Registration Procedure(s) Specification Required or IETF Review (see Note) Expert(s) Unassigned Reference [RFC-ietf-lamps-header-protection-25] Note Adding an entry to this registry with an N in the "Recommended" column follows the registration policy of SPECIFICATION REQUIRED. Adding an entry to this registry with a Y in the "Recommended" column or changing the "Recommended" column in an existing entry (from N to Y or vice versa) requires IETF REVIEW. Note The Header Confidentiality Policy Name never appears on the wire. This registry merely tracks stable references to implementable descriptions of distinct policies. Any addition to this registry should be governed by guidance in Section 3.4.2 of [RFC-ietf-lamps-header-protection-25]. Available Formats [IMG] CSV Header Confidentiality Policy Name Description Recommended Reference hcp_no_confidentiality No header confidentiality N [RFC-ietf-lamps-header-protection-25, Section 3.2.3] Confidentiality for Informational Header hcp_baseline Fields: Subject Header Field is obscured, Y [RFC-ietf-lamps-header-protection-25, Section 3.2.1] Keywords and Comments are removed Obscure Subject, remove Keywords and hcp_shy Comments, remove the time zone from Date, N [RFC-ietf-lamps-header-protection-25, Section 3.2.2] and obscure display-names Licensing Terms