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Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP)
Last Updated
2025-08-25
Note
This registry group was formerly known as "MAIL Parameters."
  
Associated Registries
[https://www.iana.org/assignments/message-headers]
  
Available Formats

XML

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Plain text

Registries Included Below

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.
    
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

CSV
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

CSV
WITH Protocol Types
Registration Procedure(s)
RFC Required
Reference
[RFC-ietf-emailcore-rfc5321bis-43]
Available Formats

CSV
Additional Registered 'Received:' Clauses
Registration Procedure(s)
IETF Review
Reference
[RFC-ietf-emailcore-rfc5321bis-43]
Available Formats

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
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 [RFC5952].
    
Available Formats

CSV
SMTP PRIORITY Extension Priority Assignment Policy
Registration Procedure(s)
Specification Required
Expert(s)
Unassigned
Reference
[RFC6710]
Available Formats

CSV
Registered-states
Registration Procedure(s)
First Come First Served
Reference
[RFC6729]
Available Formats

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

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
Registration Procedure(s)
Specification Required
Expert(s)
John Klensin, Murray Kucherawy (backup)
Reference
[RFC9422]
Available Formats

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
Registration Procedure(s)
Specification Required or IETF Review (see Note)
Expert(s)
Unassigned
Reference
[RFC9788]
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
[RFC9788].
    
Available Formats

CSV

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