Simple Mail Transfer Protocol (SMTP)

Last Updated

• 2025-09-19

Note

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

Associated Registries

Message Headers

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SMTP Service Extensions

Registration Procedure(s)

• Either Model 1 (IETF) or Model 2 (FCFS) as described in Section 8.1.1.1 of RFC-ietf-emailcore-rfc5321bis-43.

Reference

• RFC-ietf-emailcore-rfc5321bis-43, Section 8.1.1: Simple Mail Transfer Protocol, 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

SMTP Service Extension Parameters

Registration Procedure(s)

• Registry closed

Reference

• RFC-ietf-emailcore-rfc5321bis-43: Simple Mail Transfer Protocol

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, Section 8.1.3: Simple Mail Transfer Protocol, 8.1.4
• RFC-ietf-emailcore-rfc5322bis-12: Internet Message Format

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: Simple Mail Transfer Protocol

Available Formats

• CSV

WITH Protocol Types

Registration Procedure(s)

• RFC Required

Reference

• RFC-ietf-emailcore-rfc5321bis-43: Simple Mail Transfer Protocol

Available Formats

• CSV

Additional Registered 'Received:' Clauses

Registration Procedure(s)

• IETF Review

Reference

• RFC-ietf-emailcore-rfc5321bis-43: Simple Mail Transfer Protocol

Available Formats

• CSV

Address Literal Tags

Registration Procedure(s)

• Standards Action

Reference

• RFC-ietf-emailcore-rfc5321bis-43: Simple Mail Transfer Protocol

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: Simple Mail Transfer Protocol Extension for Message Transfer Priorities

Available Formats

• CSV

Registered-states

Registration Procedure(s)

• First Come First Served

Reference

• RFC 6729: Indicating Email Handling States in Trace Fields

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: Multicast Email (MULE) over Allied Communications Publication (ACP) 142

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: The LIMITS SMTP Service Extension

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: Header Protection for Cryptographically Protected Email

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