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].
See section 8.3.1 of [RFC7231] for information on registering new HTTP Header Fields.
Available Formats

Header Field Name Template Protocol Status Reference
A-IM http [RFC4229]
Accept http standard [RFC7231, Section 5.3.2]
Accept-Additions http [RFC4229]
Accept-CH http experimental [RFC-ietf-httpbis-client-hints-15, Section 3.1]
Accept-Charset http standard [RFC7231, Section 5.3.3]
Accept-Datetime http informational [RFC7089]
Accept-Encoding http standard [RFC7231, Section 5.3.4][RFC7694, Section 3]
Accept-Features http [RFC4229]
Accept-Language http standard [RFC7231, Section 5.3.5]
Accept-Language mail [RFC4021]
Accept-Patch http [RFC5789]
Accept-Post perm/accept-post http standard []
Accept-Ranges http standard [RFC7233, Section 2.3]
Age http standard [RFC7234, Section 5.1]
Allow http standard [RFC7231, Section 7.4.1]
ALPN http standard [RFC7639, Section 2]
Also-Control netnews obsoleted [RFC1849][RFC5536]
Alt-Svc http standard [RFC7838]
Alt-Used http standard [RFC7838]
Alternate-Recipient mail [RFC4021]
Alternates http [RFC4229]
Apply-To-Redirect-Ref http [RFC4437]
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-Control http experimental [RFC8053, Section 4]
Authentication-Info http standard [RFC7615, Section 3]
Authentication-Results mail standard [RFC8601]
Authorization http standard [RFC7235, Section 4.2]
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]
C-Ext http [RFC4229]
C-Man http [RFC4229]
C-Opt http [RFC4229]
C-PEP http [RFC4229]
C-PEP-Info http [RFC4229]
Cache-Control http standard [RFC7234, Section 5.2]
Cal-Managed-ID http standard [RFC8607, Section 5.1]
CalDAV-Timezones http standard [RFC7809, Section 7.1]
Cancel-Key netnews standard [RFC8315]
Cancel-Lock netnews standard [RFC8315]
Cc mail standard [RFC5322]
CDN-Loop http standard [RFC8586]
Cert-Not-After http standard [RFC8739, Section 3.3]
Cert-Not-Before http standard [RFC8739, Section 3.3]
Close http reserved [RFC7230, Section 8.1]
Comments mail standard [RFC5322]
Comments netnews standard [RFC5536][RFC5322]
Connection http standard [RFC7230, Section 6.1]
Content-Alternative MIME [RFC4021]
Content-Base http obsoleted [RFC2068][RFC2616]
Content-Base MIME obsoleted [RFC2110][RFC2557]
Content-Description MIME [RFC4021]
Content-Disposition http standard [RFC6266]
Content-Disposition MIME [RFC4021]
Content-Duration MIME [RFC4021]
Content-Encoding http standard [RFC7231, Section]
Content-features MIME [RFC4021]
Content-ID http [RFC4229]
Content-ID MIME [RFC4021]
Content-Identifier mail [RFC4021]
Content-Language http standard [RFC7231, Section]
Content-Language MIME [RFC4021]
Content-Length http standard [RFC7230, Section 3.3.2]
Content-Location http standard [RFC7231, Section]
Content-Location MIME [RFC4021]
Content-MD5 http [RFC4229]
Content-MD5 MIME [RFC4021]
Content-Range http standard [RFC7233, Section 4.2]
Content-Return mail [RFC4021]
Content-Script-Type http [RFC4229]
Content-Style-Type http [RFC4229]
Content-Transfer-Encoding MIME [RFC4021]
Content-Translation-Type MIME standard [RFC8255]
Content-Type http standard [RFC7231, Section]
Content-Type MIME [RFC4021]
Content-Version http [RFC4229]
Control netnews standard [RFC5536]
Conversion mail [RFC4021]
Conversion-With-Loss mail [RFC4021]
Cookie http standard [RFC6265]
Cookie2 http obsoleted [RFC2965][RFC6265]
DASL http standard [RFC5323]
DAV http standard [RFC4918]
DL-Expansion-History mail [RFC4021]
Date http standard [RFC7231, Section]
Date mail standard [RFC5322]
Date netnews standard [RFC5536][RFC5322]
Date-Received netnews obsoleted [RFC0850][RFC5536]
Default-Style http [RFC4229]
Deferred-Delivery mail [RFC4021]
Delivery-Date mail [RFC4021]
Delta-Base http [RFC4229]
Depth http standard [RFC4918]
Derived-From http [RFC4229]
Destination http standard [RFC4918]
Differential-ID http [RFC4229]
Digest http [RFC4229]
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]
Early-Data http standard [RFC8470]
Encoding mail [RFC4021]
Encrypted mail [RFC4021]
ETag http standard [RFC7232, Section 2.3]
Expect http standard [RFC7231, Section 5.1.1]
Expect-CT http experimental [RFC-ietf-httpbis-expect-ct-08]
Expires http standard [RFC7234, Section 5.3]
Expires mail [RFC4021]
Expires netnews standard [RFC5536]
Expiry-Date mail [RFC4021]
Ext http [RFC4229]
Followup-To netnews standard [RFC5536]
Forwarded http standard [RFC7239]
From http standard [RFC7231, Section 5.5.1]
From mail standard [RFC5322][RFC6854]
From netnews standard [RFC5536][RFC5322]
Generate-Delivery-Report mail [RFC4021]
GetProfile http [RFC4229]
Hobareg http experimental [RFC7486, Section 6.1.1]
Host http standard [RFC7230, Section 5.4]
HTTP2-Settings http standard [RFC7540, Section 3.2.1]
IM http [RFC4229]
If http standard [RFC4918]
If-Match http standard [RFC7232, Section 3.1]
If-Modified-Since http standard [RFC7232, Section 3.3]
If-None-Match http standard [RFC7232, Section 3.2]
If-Range http standard [RFC7233, Section 3.2]
If-Schedule-Tag-Match http standard [RFC6638]
If-Unmodified-Since http standard [RFC7232, Section 3.4]
Importance mail [RFC4021]
In-Reply-To mail standard [RFC5322]
Include-Referred-Token-Binding-ID http standard [RFC8473]
Incomplete-Copy mail [RFC4021]
Injection-Date netnews standard [RFC5536]
Injection-Info netnews standard [RFC5536]
Keep-Alive http [RFC4229]
Keywords mail standard [RFC5322]
Keywords netnews standard [RFC5536][RFC5322]
Label http [RFC4229]
Language mail [RFC4021]
Last-Modified http standard [RFC7232, Section 2.2]
Latest-Delivery-Time mail [RFC4021]
Lines netnews deprecated [RFC5536][RFC3977]
Link http standard [RFC8288]
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]
Location http standard [RFC7231, Section 7.1.2]
Lock-Token http standard [RFC4918]
Man http [RFC4229]
Max-Forwards http standard [RFC7231, Section 5.1.2]
Memento-Datetime http informational [RFC7089]
Message-Context mail [RFC4021]
Message-ID mail standard [RFC5322]
Message-ID netnews standard [RFC5536][RFC5322]
Message-Type mail [RFC4021]
Meter http [RFC4229]
MIME-Version http standard [RFC7231, Appendix A.1]
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]
Negotiate http [RFC4229]
Newsgroups netnews standard [RFC5536]
NNTP-Posting-Date netnews obsoleted [RFC5536]
NNTP-Posting-Host netnews obsoleted [RFC2980][RFC5536]
Obsoletes mail [RFC4021]
OData-EntityId perm/odata-entityid http standard [OData Version 4.01 Part 1: Protocol][OASIS][Chet_Ensign]
OData-Isolation perm/odata-isolation http standard [OData Version 4.01 Part 1: Protocol][OASIS][Chet_Ensign]
OData-MaxVersion perm/odata-maxversion http standard [OData Version 4.01 Part 1: Protocol][OASIS][Chet_Ensign]
OData-Version perm/odata-version http standard [OData Version 4.01 Part 1: Protocol][OASIS][Chet_Ensign]
Opt http [RFC4229]
Optional-WWW-Authenticate http experimental [RFC8053, Section 3]
Ordering-Type http standard [RFC4229]
Organization mail informational [RFC7681]
Organization netnews standard [RFC5536]
Origin http standard [RFC6454]
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]
OSCORE http standard [RFC8613, Section 11.1]
Overwrite http standard [RFC4918]
P3P http [RFC4229]
Path netnews standard [RFC5536]
PEP http [RFC4229]
PICS-Label http [RFC4229]
PICS-Label mail [RFC4021]
Pep-Info http [RFC4229]
Position http standard [RFC4229]
Posting-Version netnews obsoleted [RFC0850][RFC5536]
Pragma http standard [RFC7234, Section 5.4]
Prefer http standard [RFC7240]
Preference-Applied http standard [RFC7240]
Prevent-NonDelivery-Report mail [RFC4021]
Priority mail [RFC4021]
ProfileObject http [RFC4229]
Protocol http [RFC4229]
Protocol-Info http [RFC4229]
Protocol-Query http [RFC4229]
Protocol-Request http [RFC4229]
Proxy-Authenticate http standard [RFC7235, Section 4.3]
Proxy-Authentication-Info http standard [RFC7615, Section 4]
Proxy-Authorization http standard [RFC7235, Section 4.4]
Proxy-Features http [RFC4229]
Proxy-Instruction http [RFC4229]
Public http [RFC4229]
Public-Key-Pins http standard [RFC7469]
Public-Key-Pins-Report-Only http standard [RFC7469]
Range http standard [RFC7233, Section 3.1]
Received mail standard [RFC5322][RFC5321]
Received-SPF mail standard [RFC7208]
Redirect-Ref http [RFC4437]
References mail standard [RFC5322]
References netnews standard [RFC5536][RFC5322]
Referer http standard [RFC7231, Section 5.5.2]
Relay-Version netnews obsoleted [RFC0850][RFC5536]
Replay-Nonce http standard [RFC8555, Section 6.5.1]
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]
Retry-After http standard [RFC7231, Section 7.1.3]
Return-Path mail standard [RFC5322]
Safe http [RFC4229]
Schedule-Reply http standard [RFC6638]
Schedule-Tag http standard [RFC6638]
Sec-Token-Binding http standard [RFC8473]
Sec-WebSocket-Accept http standard [RFC6455]
Sec-WebSocket-Extensions http standard [RFC6455]
Sec-WebSocket-Key http standard [RFC6455]
Sec-WebSocket-Protocol http standard [RFC6455]
Sec-WebSocket-Version http standard [RFC6455]
Security-Scheme http [RFC4229]
See-Also netnews obsoleted [RFC1849][RFC5536]
Sender mail standard [RFC5322][RFC6854]
Sender netnews standard [RFC5536][RFC5322]
Sensitivity mail [RFC4021]
Server http standard [RFC7231, Section 7.4.2]
Set-Cookie http standard [RFC6265]
Set-Cookie2 http obsoleted [RFC2965][RFC6265]
SetProfile http [RFC4229]
SLUG http standard [RFC5023]
SoapAction http [RFC4229]
Solicitation mail [RFC3865]
Status-URI http [RFC4229]
Strict-Transport-Security http standard [RFC6797]
Subject mail standard [RFC5322]
Subject netnews standard [RFC5536][RFC5322]
Summary netnews standard [RFC5536]
Sunset http informational [RFC8594]
Supersedes mail [RFC4021]
Supersedes netnews standard [RFC5536][RFC2156]
Surrogate-Capability http [RFC4229]
Surrogate-Control http [RFC4229]
TCN http [RFC4229]
TE http standard [RFC7230, Section 4.3]
Timeout http standard [RFC4918]
TLS-Report-Domain mail standard [RFC8460]
TLS-Report-Submitter mail standard [RFC8460]
TLS-Required mail standard [RFC8689]
To mail standard [RFC5322]
Topic http standard [RFC8030, Section 5.4]
Trailer http standard [RFC7230, Section 4.4]
Transfer-Encoding http standard [RFC7230, Section 3.3.1]
TTL http standard [RFC8030, Section 5.2]
Urgency http standard [RFC8030, Section 5.3]
URI http [RFC4229]
Upgrade http standard [RFC7230, Section 6.7]
User-Agent http standard [RFC7231, Section 5.5.3]
User-Agent netnews standard [RFC5536][RFC2616]
Variant-Vary http [RFC4229]
Vary http standard [RFC7231, Section 7.1.4]
VBR-Info mail standard [RFC5518]
Via http standard [RFC7230, Section 5.7.1]
WWW-Authenticate http standard [RFC7235, Section 4.1]
Want-Digest http [RFC4229]
Warning http standard [RFC7234, Section 5.5]
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]
X-Content-Type-Options http standard []
X-Frame-Options http informational [RFC7034]
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.
Available Formats

Header Field Name Template Protocol Status Reference
Access-Control prov/access-control http deprecated [W3C Web Application Formats Working Group]
Access-Control-Allow-Credentials prov/access-control-allow-credentials http [W3C Web Application Formats Working Group]
Access-Control-Allow-Headers prov/access-control-allow-headers http [W3C Web Application Formats Working Group]
Access-Control-Allow-Methods prov/access-control-allow-methods http [W3C Web Application Formats Working Group]
Access-Control-Allow-Origin prov/access-control-allow-origin http [W3C Web Application Formats Working Group]
Access-Control-Max-Age prov/access-control-max-age http [W3C Web Application Formats Working Group]
Access-Control-Request-Method prov/access-control-request-method http [W3C Web Application Formats Working Group]
Access-Control-Request-Headers prov/access-control-request-headers http [W3C Web Application Formats Working Group]
AMP-Cache-Transform http []
Apparently-To prov/apparently-to mail [RFC2076]
Compliance http [RFC4229]
Content-Transfer-Encoding http [RFC4229]
Cost http [RFC4229]
EDIINT-Features http [RFC6017]
EDIINT-Features mail [RFC6017]
Eesst-Version mail [RFC7681]
Errors-To prov/errors-to mail [RFC2076]
Form-Sub mail [draft-levine-mailbomb-header]
Isolation prov/isolation http [OData Version 4.01 Part 1: Protocol][OASIS][Chet_Ensign]
Jabber-ID prov/jabber-id mail [RFC7259]
Jabber-ID prov/jabber-id netnews [RFC7259]
Message-ID http [RFC4229]
Method-Check prov/method-check http deprecated [W3C Web Application Formats Working Group]
Method-Check-Expires prov/method-check-expires http deprecated [W3C Web Application Formats Working Group]
MMHS-Authorizing-Users mail [RFC7912]
Non-Compliance http [RFC4229]
Optional http [RFC4229]
Privicon mail [draft-koenig-privicons]
Referer-Root prov/referer-root http deprecated [W3C Web Application Formats Working Group]
Repeatability-Client-ID prov/repeatability-client-id http [Repeatable Requests Version 1.0][OASIS][Chet_Ensign]
Repeatability-First-Sent prov/repeatability-first-sent http [Repeatable Requests Version 1.0][OASIS][Chet_Ensign]
Repeatability-Request-ID prov/repeatability-request-id http [Repeatable Requests Version 1.0][OASIS][Chet_Ensign]
Repeatability-Result prov/repeatability-result http [Repeatable Requests Version 1.0][OASIS][Chet_Ensign]
Resolution-Hint http [RFC4229]
Resolver-Location http [RFC4229]
SIO-Label mail [RFC7444]
SIO-Label-History mail [RFC7444]
SubOK http [RFC4229]
Subst http [RFC4229]
Timing-Allow-Origin prov/timing-allow-origin http []
Title http [RFC4229]
Traceparent prov/traceparent http []
Tracestate prov/tracestate http []
UA-Color http [RFC4229]
UA-Media http [RFC4229]
UA-Pixels http [RFC4229]
UA-Resolution http [RFC4229]
UA-Windowpixels http [RFC4229]
Version http [RFC4229]
X-Archived-At prov/x-archived-at mail deprecated [RFC5064]
X-Archived-At prov/x-archived-at netnews deprecated [RFC5064]
X-Device-Accept prov/x-device-accept http [W3C Mobile Web Best Practices Working Group]
X-Device-Accept-Charset prov/x-device-accept-charset http [W3C Mobile Web Best Practices Working Group]
X-Device-Accept-Encoding prov/x-device-accept-encoding http [W3C Mobile Web Best Practices Working Group]
X-Device-Accept-Language prov/x-device-accept-language http [W3C Mobile Web Best Practices Working Group]
X-Device-User-Agent prov/x-device-user-agent http [W3C Mobile Web Best Practices Working Group]
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
Nathaniel Borenstein, 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]

Contact Information

ID Name Contact URI Last Updated
[Chet_Ensign] Chet Ensign mailto:chet.ensign& 2020-09-01