Simple Mail Transfer Protocol (SMTP)

Last Updated

2025-09-19

Note

This registry group was formerly known as "MAIL Parameters."
  

Associated Registries

[https://www.iana.org/assignments/message-headers]
  

Available Formats

XML
HTML
TXT

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 [RFC 6409]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

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.
    

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 [RFC 5952].
    

Available Formats

CSV

SMTP PRIORITY Extension Priority Assignment Policy

Registration Procedure(s)

Specification Required

Expert(s)

Unassigned

Reference

[RFC 6710]

Available Formats

CSV

Registered-states

Registration Procedure(s)

First Come First Served

Reference

[RFC 6729]

Available Formats

CSV

Multicast Email SMTP Extensions

Registration Procedure(s)

Specification Required

Expert(s)

Alexey Melnikov (primary), RFC Independent Submissions Editor (secondary)

Reference

[RFC 8494]

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

SMTP Server Limits

Registration Procedure(s)

Specification Required

Expert(s)

John Klensin, Murray Kucherawy (backup)

Reference

[RFC 9422]

Available Formats

CSV

Mail Header Confidentiality Policies

Registration Procedure(s)

Specification Required or IETF Review (see Note)

Expert(s)

Daniel Kahn Gillmor (primary), Bernie Hoeneisen (secondary)

Reference

[RFC 9788]

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 9788].
    

Available Formats

CSV