Message Headers

Last Updated
2025年08月25日
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][RFC9788]
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 [RFC9788 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][RFC-ietf-emailcore-rfc5321bis-43]
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][RFC-ietf-emailcore-rfc5321bis-43]
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 Trace 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]

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