Showing content from http://www.iana.org/assignments/address-literal-tags below:
Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP)
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Last Updated
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2025-08-25
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Note
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This registry group was formerly known as "MAIL Parameters."
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Associated Registries
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[https://www.iana.org/assignments/message-headers]
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Available Formats
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XML
HTML
Plain text
Registries Included Below
SMTP Service Extensions
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Registration Procedure(s)
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Either Model 1 (IETF) or Model 2 (FCFS) as described in Sections 8.1.1.1 of [RFC-ietf-emailcore-rfc5321bis-43].
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.1, 2.2.2]
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Note
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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.
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Note
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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.
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Available Formats
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CSV
SMTP Service Extension Parameters
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Registration Procedure(s)
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Registry closed
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43]
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Note
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These entries have been incorporated into the SMTP Service Extension registry's
"EHLO Parameters" field.
Mail Transmission Types for the "Received:" Header Field
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.3, 8.1.4][RFC-ietf-emailcore-rfc5322bis-12]
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Note
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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
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Registration Procedure(s)
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RFC Required
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43]
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Available Formats
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CSV
WITH Protocol Types
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Registration Procedure(s)
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RFC Required
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43]
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Available Formats
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CSV
Additional Registered 'Received:' Clauses
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Registration Procedure(s)
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IETF Review
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43]
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Available Formats
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CSV
Clause Name Service Extension Description Syntax Summary Reference PRIORITY Records the value of the MT-PRIORITY parameter specified in the MAIL FROM command See Section 7 of RFC [RFC6710] [RFC6710] state Indicates entry into a special queue state state <state-name> [RFC6729] tls Indicates the TLS cipher used tls-cipher = tls-cipher-name / tls-cipher-hex
tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
; as registered in the IANA TLS Cipher Suite Registry
tls-cipher-hex = "0x" 4HEXDIG
[RFC8314] group Indicates the Diffie-Hellman group used with the TLS cipher (if applicable) dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-")
; as registered in the IANA TLS Supported Groups Registry [RFC8314] Address Literal Tags
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Registration Procedure(s)
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Standards Action
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Reference
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[RFC-ietf-emailcore-rfc5321bis-43]
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Note
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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 [RFC5952].
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Available Formats
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CSV
SMTP PRIORITY Extension Priority Assignment Policy
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Registration Procedure(s)
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Specification Required
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Expert(s)
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Unassigned
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Reference
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[RFC6710]
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Available Formats
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CSV
Registered-states
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Registration Procedure(s)
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First Come First Served
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Reference
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[RFC6729]
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Available Formats
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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
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Registration Procedure(s)
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Specification Required
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Expert(s)
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Alexey Melnikov (primary), RFC Independent Submissions Editor (secondary)
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Reference
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[RFC8494]
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Note
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Extension names/keywords and status used with Multicast Email; see
SMTP Service Extensions above for the list for more general use.
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Available Formats
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CSV
Name Status Multicast-Mail-Specific References Non-Multicast-Specific References Note SIZE Required [RFC1870] 8BITMIME Required [RFC6152] DSN Required [RFC3461] MT-PRIORITY Required [RFC6710] DELIVERBY Required [RFC2852] BINARYMIME Required [RFC3030] CHUNKING Special [RFC3030] SMTP CHUNKING MUST be supported on the receiving SMTP 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. 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 [Moving RFC 4405, RFC 4406, RFC 4407 (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
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Registration Procedure(s)
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Specification Required
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Expert(s)
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John Klensin, Murray Kucherawy (backup)
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Reference
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[RFC9422]
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Available Formats
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CSV
Name Value syntax Description Restrictions Security Considerations Reference MAILMAX %x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum MAILMAX specifies the maximum number of transactions (MAIL FROM commands) the server will accept in a single session. The count includes all MAIL FROM commands, regardless of whether they succeed or fail. None See [RFC9422, Section 6] [RFC9422] RCPTMAX %x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum RCPTMAX specifies the maximum number of RCPT TO commands the server will accept in a single transaction. It is not a limit on the actual number of 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. None See [RFC9422, Section 6] [RFC9422] RCPTDOMAINMAX %x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum RCPTDOMAINMAX specifies the maximum number of different domains that can appear in a recipient (RCPT TO) address within a single session. This limit is imposed by some servers that bind to a specific internal delivery mechanism on receipt of the first RCPT TO command. None See [RFC9422, Section 6] [RFC9422] Mail Header Confidentiality Policies
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Registration Procedure(s)
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Specification Required or IETF Review (see Note)
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Expert(s)
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Unassigned
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Reference
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[RFC9788]
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Note
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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.
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Note
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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
[RFC9788].
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Available Formats
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CSV
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