Internet Assigned Numbers Authority

Message Headers

Last Updated
Available Formats



Plain text

Registries included below

Permanent Message Header Field Names

Registration Procedure(s)
Expert Review
Graham Klyne
[RFC5504] specified that no new header fields be registered that begin with "Downgraded-". 
That restriction is now lifted, per [RFC6857].
HTTP field name registrations have been moved to [] 
per [RFC9110].
Available Formats

Header Field Name Template Protocol Status 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 [RFC5322]
Body none reserved [RFC6068]
Cancel-Key netnews standard [RFC8315]
Cancel-Lock netnews standard [RFC8315]
Cc mail standard [RFC5322]
Comments mail standard [RFC5322]
Comments netnews standard [RFC5536][RFC5322]
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]
Control netnews standard [RFC5536]
Conversion mail [RFC4021]
Conversion-With-Loss mail [RFC4021]
DL-Expansion-History mail [RFC4021]
Date mail standard [RFC5322]
Date netnews standard [RFC5536][RFC5322]
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 [RFC5322][RFC6854]
From netnews standard [RFC5536][RFC5322]
Generate-Delivery-Report mail [RFC4021]
Importance mail [RFC4021]
In-Reply-To mail standard [RFC5322]
Incomplete-Copy mail [RFC4021]
Injection-Date netnews standard [RFC5536]
Injection-Info netnews standard [RFC5536]
Keywords mail standard [RFC5322]
Keywords netnews standard [RFC5536][RFC5322]
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 [RFC5322]
Message-ID netnews standard [RFC5536][RFC5322]
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 [RFC5322][RFC5321]
Received-SPF mail standard [RFC7208]
References mail standard [RFC5322]
References netnews standard [RFC5536][RFC5322]
Relay-Version netnews obsoleted [RFC850][RFC5536]
Reply-By mail [RFC4021]
Reply-To mail standard [RFC5322]
Reply-To netnews standard [RFC5536][RFC5322]
Require-Recipient-Valid-Since mail standard [RFC7293]
Resent-Bcc mail standard [RFC5322]
Resent-Cc mail standard [RFC5322]
Resent-Date mail standard [RFC5322]
Resent-From mail standard [RFC5322][RFC6854]
Resent-Message-ID mail standard [RFC5322]
Resent-Reply-To mail obsoleted [RFC5322]
Resent-Sender mail standard [RFC5322][RFC6854]
Resent-To mail standard [RFC5322]
Return-Path mail standard [RFC5322]
See-Also netnews obsoleted [RFC1849][RFC5536]
Sender mail standard [RFC5322][RFC6854]
Sender netnews standard [RFC5536][RFC5322]
Sensitivity mail [RFC4021]
Solicitation mail [RFC3865]
Subject mail standard [RFC5322]
Subject netnews standard [RFC5536][RFC5322]
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 [RFC5322]
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
Graham Klyne
Registration of a Provisional Message Header Field does not of itself imply any kind of 
endorsement by the IETF, IANA or any other body.
HTTP field name registrations have been moved to [] 
per [RFC-ietf-httpbis-semantics-19].
Available Formats

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]
Form-Sub mail [draft-levine-mailbomb-header]
Jabber-ID prov/jabber-id mail [RFC7259]
Jabber-ID prov/jabber-id netnews [RFC7259]
MMHS-Authorizing-Users mail [RFC7912]
Privicon mail [draft-koenig-privicons]
SIO-Label mail [RFC7444]
SIO-Label-History mail [RFC7444]
Wrong-Recipient prov/wrong-recipient mail [draft-dweekly-wrong-recipient-04]
X-Archived-At prov/x-archived-at mail deprecated [RFC5064]
X-Archived-At prov/x-archived-at netnews deprecated [RFC5064]
X-Mittente mail [RFC6109]
X-PGP-Sig prov/x-pgp-sig netnews [][]
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
Nik Tomkinson
Available Formats

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]