Message Headers

Last Updated: 2025-04-17
Available Formats: XML
HTML
Plain text

Registries included below

Permanent Message Header Field Names

Registration Procedure(s)

Expert Review

Expert(s)

Mark Baker (primary), Murray Kucherawy (backup)

Reference

[RFC3864][RFC-ietf-emailcore-rfc5322bis-12]

Note

[RFC5504] specified that no new header fields be registered that begin with "Downgraded-". That restriction is now lifted, per [RFC6857].

Note

HTTP field name registrations have been moved to [https://www.iana.org/assignments/http-fields] per [RFC9110].

Available Formats

CSV

Header Field Name	Template	Protocol	Status	Trace	Reference
Accept-Language		mail			[RFC4021]
Also-Control		netnews	obsoleted		[RFC1849][RFC5536]
Alternate-Recipient		mail			[RFC4021]
Approved		netnews	standard		[RFC5536]
ARC-Authentication-Results		mail	experimental		[RFC8617]
ARC-Message-Signature		mail	experimental		[RFC8617]
ARC-Seal		mail	experimental		[RFC8617]
Archive		netnews	standard		[RFC5536]
Archived-At		mail	standard		[RFC5064]
Archived-At		netnews	standard		[RFC5064]
Article-Names		netnews	obsoleted		[RFC1849][RFC5536]
Article-Updates		netnews	obsoleted		[RFC1849][RFC5536]
Authentication-Results		mail	standard		[RFC8601]
Auto-Submitted		mail	standard		[RFC3834 section 5]
Autoforwarded		mail			[RFC4021]
Autosubmitted		mail			[RFC4021]
Base		MIME	obsoleted		[RFC1808][RFC2068 Section 14.11]
Bcc		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.3]
Body		none	reserved		[RFC6068]
Cancel-Key		netnews	standard		[RFC8315]
Cancel-Lock		netnews	standard		[RFC8315]
Cc		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.3]
Comments		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.5]
Comments		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Content-Alternative		MIME			[RFC4021]
Content-Base		MIME	obsoleted		[RFC2110][RFC2557]
Content-Description		MIME			[RFC4021]
Content-Disposition		MIME			[RFC4021]
Content-Duration		MIME			[RFC4021]
Content-features		MIME			[RFC4021]
Content-ID		MIME			[RFC4021]
Content-Identifier		mail			[RFC4021]
Content-Language		MIME			[RFC4021]
Content-Location		MIME			[RFC4021]
Content-MD5		MIME			[RFC4021]
Content-Return		mail			[RFC4021]
Content-Transfer-Encoding		MIME			[RFC4021]
Content-Translation-Type		MIME	standard		[RFC8255]
Content-Type		MIME			[RFC4021][RFC-ietf-lamps-header-protection-25]
Control		netnews	standard		[RFC5536]
Conversion		mail			[RFC4021]
Conversion-With-Loss		mail			[RFC4021]
DL-Expansion-History		mail			[RFC4021]
Date		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.1]
Date		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Date-Received		netnews	obsoleted		[RFC850][RFC5536]
Deferred-Delivery		mail			[RFC4021]
Delivery-Date		mail			[RFC4021]
Discarded-X400-IPMS-Extensions		mail			[RFC4021]
Discarded-X400-MTS-Extensions		mail			[RFC4021]
Disclose-Recipients		mail			[RFC4021]
Disposition-Notification-Options		mail			[RFC4021]
Disposition-Notification-To		mail			[RFC4021]
Distribution		netnews	standard		[RFC5536]
DKIM-Signature		mail	standard		[RFC6376]
Downgraded-Bcc		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Cc		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Disposition-Notification-To		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Final-Recipient		mail	standard		[RFC6857 Section 3.1.10]
Downgraded-From		mail	obsoleted		[RFC5504][RFC6857 Section 3.1.10]
Downgraded-In-Reply-To		mail	standard		[RFC6857 Section 3.1.10]
Downgraded-Mail-From		mail	obsoleted		[RFC5504][RFC6857 Section 3.1.10]
Downgraded-Message-Id		mail	standard		[RFC6857 Section 3.1.10]
Downgraded-Original-Recipient		mail	standard		[RFC6857 Section 3.1.10]
Downgraded-Rcpt-To		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-References		mail	standard		[RFC6857 Section 3.1.10]
Downgraded-Reply-To		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-Bcc		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-Cc		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-From		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-Reply-To		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-Sender		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Resent-To		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Return-Path		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-Sender		mail	obsoleted		[RFC5504][RFC6857]
Downgraded-To		mail	obsoleted		[RFC5504][RFC6857]
Encoding		mail			[RFC4021]
Encrypted		mail			[RFC4021]
Expires		mail			[RFC4021]
Expires		netnews	standard		[RFC5536]
Expiry-Date		mail			[RFC4021]
Followup-To		netnews	standard		[RFC5536]
From		mail	standard	no	[RFC6854][RFC-ietf-emailcore-rfc5322bis-12, 3.6.2]
From		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Generate-Delivery-Report		mail			[RFC4021]
HP-Outer		mail	standard		[RFC-ietf-lamps-header-protection-25 Section 2.2.1]
Importance		mail			[RFC4021]
In-Reply-To		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.4]
Incomplete-Copy		mail			[RFC4021]
Injection-Date		netnews	standard		[RFC5536]
Injection-Info		netnews	standard		[RFC5536]
Keywords		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.5]
Keywords		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Language		mail			[RFC4021]
Latest-Delivery-Time		mail			[RFC4021]
Lines		netnews	deprecated		[RFC5536][RFC3977]
List-Archive		mail			[RFC4021]
List-Help		mail			[RFC4021]
List-ID		mail			[RFC4021]
List-Owner		mail			[RFC4021]
List-Post		mail			[RFC4021]
List-Subscribe		mail			[RFC4021]
List-Unsubscribe	perm/list-unsubscribe	mail			[RFC4021]
List-Unsubscribe-Post		mail	standard		[RFC8058]
Message-Context		mail			[RFC4021]
Message-ID		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.4]
Message-ID		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Message-Type		mail			[RFC4021]
MIME-Version		MIME			[RFC4021]
MMHS-Exempted-Address		mail			[RFC6477][ACP123 Appendix A1.1 and Appendix B.105]
MMHS-Extended-Authorisation-Info		mail			[RFC6477][ACP123 Appendix A1.2 and Appendix B.106]
MMHS-Subject-Indicator-Codes		mail			[RFC6477][ACP123 Appendix A1.3 and Appendix B.107]
MMHS-Handling-Instructions		mail			[RFC6477][ACP123 Appendix A1.4 and Appendix B.108]
MMHS-Message-Instructions		mail			[RFC6477][ACP123 Appendix A1.5 and Appendix B.109]
MMHS-Codress-Message-Indicator		mail			[RFC6477][ACP123 Appendix A1.6 and Appendix B.110]
MMHS-Originator-Reference		mail			[RFC6477][ACP123 Appendix A1.7 and Appendix B.111]
MMHS-Primary-Precedence		mail			[RFC6477][ACP123 Appendix A1.8 and Appendix B.101]
MMHS-Copy-Precedence		mail			[RFC6477][ACP123 Appendix A1.9 and Appendix B.102]
MMHS-Message-Type		mail			[RFC6477][ACP123 Appendix A1.10 and Appendix B.103]
MMHS-Other-Recipients-Indicator-To		mail			[RFC6477][ACP123 Appendix A1.12 and Appendix B.113]
MMHS-Other-Recipients-Indicator-CC		mail			[RFC6477][ACP123 Appendix A1.12 and Appendix B.113]
MMHS-Acp127-Message-Identifier		mail			[RFC6477][ACP123 Appendix A1.14 and Appendix B.116]
MMHS-Originator-PLAD		mail			[RFC6477][ACP123 Appendix A1.15 and Appendix B.117]
MT-Priority		mail	standard		[RFC6758]
Newsgroups		netnews	standard		[RFC5536]
NNTP-Posting-Date		netnews	obsoleted		[RFC5536]
NNTP-Posting-Host		netnews	obsoleted		[RFC2980][RFC5536]
Obsoletes		mail			[RFC4021]
Organization		mail	informational		[RFC7681]
Organization		netnews	standard		[RFC5536]
Original-Encoded-Information-Types		mail			[RFC4021]
Original-From		mail	standard		[RFC5703]
Original-Message-ID		mail			[RFC4021]
Original-Recipient	perm/original-recipient	mail	standard		[RFC3798][RFC5337]
Original-Sender		netnews	standard		[RFC5537]
Originator-Return-Address		mail			[RFC4021]
Original-Subject		mail	standard		[RFC5703]
Path		netnews	standard		[RFC5536]
PICS-Label		mail			[RFC4021]
Posting-Version		netnews	obsoleted		[RFC850][RFC5536]
Prevent-NonDelivery-Report		mail			[RFC4021]
Priority		mail			[RFC4021]
Received		mail	standard	yes	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.7][draft-ietf-emailcore-rfc5321bis-42]
Received-SPF		mail	standard		[RFC7208]
References		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.4]
References		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Relay-Version		netnews	obsoleted		[RFC850][RFC5536]
Reply-By		mail			[RFC4021]
Reply-To		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.2]
Reply-To		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Require-Recipient-Valid-Since		mail	standard		[RFC7293]
Resent-Bcc		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-Cc		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-Date		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-From		mail	standard	no	[RFC6854][RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-Message-ID		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-Reply-To		mail	obsoleted	no	[RFC-ietf-emailcore-rfc5322bis-12, 4.5.6]
Resent-Sender		mail	standard	no	[RFC6854][RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Resent-To		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.6]
Return-Path		mail	standard	yes	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.7][draft-ietf-emailcore-rfc5321bis-42]
See-Also		netnews	obsoleted		[RFC1849][RFC5536]
Sender		mail	standard	no	[RFC6854][RFC-ietf-emailcore-rfc5322bis-12, 3.6.2]
Sender		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Sensitivity		mail			[RFC4021]
Solicitation		mail			[RFC3865]
Subject		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.5]
Subject		netnews	standard		[RFC5536][RFC-ietf-emailcore-rfc5322bis-12]
Summary		netnews	standard		[RFC5536]
Supersedes		mail			[RFC4021]
Supersedes		netnews	standard		[RFC5536][RFC2156]
TLS-Report-Domain		mail	standard		[RFC8460]
TLS-Report-Submitter		mail	standard		[RFC8460]
TLS-Required		mail	standard		[RFC8689]
To		mail	standard	no	[RFC-ietf-emailcore-rfc5322bis-12, 3.6.3]
User-Agent		netnews	standard		[RFC5536][RFC2616]
VBR-Info		mail	standard		[RFC5518]
X400-Content-Identifier		mail			[RFC4021]
X400-Content-Return		mail			[RFC4021]
X400-Content-Type		mail			[RFC4021]
X400-MTS-Identifier		mail			[RFC4021]
X400-Originator		mail			[RFC4021]
X400-Received		mail			[RFC4021]
X400-Recipients		mail			[RFC4021]
X400-Trace		mail			[RFC4021]
Xref		netnews	standard		[RFC5536]

Provisional Message Header Field Names

Registration Procedure(s)

Expert Review

Expert(s)

Mark Baker (primary), Murray Kucherawy (backup)

Reference

[RFC3864][RFC-ietf-emailcore-rfc5322bis-12]

Note

Registration of a Provisional Message Header Field does not of itself imply any kind of endorsement by the IETF, IANA or any other body.

Note

HTTP field name registrations have been moved to [https://www.iana.org/assignments/http-fields] per [RFC9110].

Available Formats

CSV

Header Field Name	Template	Protocol	Status	Reference
Apparently-To	prov/apparently-to	mail		[RFC2076]
Author		mail		[RFC9057]
CFBL-Address		mail		[RFC9477]
CFBL-Feedback-ID		mail		[RFC9477]
Delivered-To		mail		[RFC9228]
EDIINT-Features		mail		[RFC6017]
Eesst-Version		mail		[RFC7681]
Errors-To	prov/errors-to	mail		[RFC2076]
Face	prov/face	mail		[https://quimby.gnus.org/circus/face]
Face	prov/face	netnews		[https://quimby.gnus.org/circus/face]
Form-Sub		mail		[draft-levine-mailbomb-header-00]
Jabber-ID	prov/jabber-id	mail		[RFC7259]
Jabber-ID	prov/jabber-id	netnews		[RFC7259]
MMHS-Authorizing-Users		mail		[RFC7912]
Privicon		mail		[draft-koenig-privicons-01]
SIO-Label		mail		[RFC7444]
SIO-Label-History		mail		[RFC7444]
Wrong-Recipient	prov/wrong-recipient	mail		[draft-ietf-mailmaint-wrong-recipient-00]
X-Archived-At	prov/x-archived-at	mail	deprecated	[RFC5064]
X-Archived-At	prov/x-archived-at	netnews	deprecated	[RFC5064]
X-Face	prov/x-face	mail		[https://purl.org/x-face-spec]
X-Face	prov/x-face	netnews		[https://purl.org/x-face-spec]
X-Mittente		mail		[RFC6109]
X-PGP-Sig	prov/x-pgp-sig	netnews		[ftp://ftp.isc.org/pub/pgpcontrol/FORMAT][https://ftp.isc.org/pub/pgpcontrol/FORMAT]
X-Ricevuta		mail		[RFC6109]
X-Riferimento-Message-ID		mail		[RFC6109]
X-TipoRicevuta		mail		[RFC6109]
X-Trasporto		mail		[RFC6109]
X-VerificaSicurezza		mail		[RFC6109]

Content-Translation-Type Header Field Values

Registration Procedure(s)

Specification Required

Expert(s)

Nik Tomkinson

Reference

[RFC8255]

Available Formats

CSV

Value	Description	Reference
original	Content in the original language	[RFC8255]
human	Content that has been translated by a human translator or a human has checked and corrected an automated translation	[RFC8255]
automated	Content that has been translated by an electronic agent without proofreading or subsequent correction	[RFC8255]