Message Headers

Message Headers

 

An email consists of three vital components: the envelope, the header(s), and the body of the message. The envelope is something that an email user will never see since it is part of the internal process by which an email is routed. The body is the part that we always see as it is the actual content of the message contained in the email. The header(s), the third component of an email, is perhaps a little more difficult to explain, though it is arguably the most interesting part of an email.

In an e-mail, the body (content text) is always preceded by header lines that identify particular routing information of the message, including the sender, recipient, date and subject. Some headers are mandatory, such as the FROM, TO and DATE headers. Others are optional, but very commonly used, such as SUBJECT and CC. Other headers include the sending time stamps and the receiving time stamps of all mail transfer agents that have received and sent the message. In other words, any time a message is transferred from one user to another (i.e. when it is sent or forwarded), the message is date/time stamped by a mail transfer agent (MTA) – a computer program or software agent that facilitates the transfer of email message from one computer to another. This date/time stamp, like FROM, TO, and SUBJECT, becomes one of the many headers that precede the body of an email.

 

Permanent Message Header Field Names

 
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 [RFC0850][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 [RFC0850][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 [RFC0850][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

Header Field Name Template Protocol Status Reference
Apparently-To prov/apparently-to mail   [RFC2076]
Author   mail   [RFC9057]
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]
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   [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

 
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]

 

Message Headers
What is an Email Header?