Simple Mail Transfer Protocol (SMTP) (original) (raw)

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 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.

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

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

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

SMTP Server Limits

Registration Procedure(s)

Specification Required

Expert(s)

John Klensin, Murray Kucherawy (backup)

Reference

[RFC9422]

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

[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