Editing Email (section)

===Message header===
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RFC 5322 specifies the [[syntax]] of the email header. Each email message has a [[Header (computing)|header]] (the "header section" of the message, according to the specification), comprising a number of [[Field (computer science)|fields]] ("header fields"). Each field has a name ("field name" or "header field name"), followed by the separator character ":", and a value ("field body" or "header field body").

Each field name begins in the first character of a new line in the header section, and begins with a non-[[Whitespace character|whitespace]] [[Printable characters|printable character]]. It ends with the separator character ":". The separator is followed by the field value (the "field body"). The value can continue onto subsequent lines if those lines have space or tab as their first character. Field names and, without [[Simple Mail Transfer Protocol#SMTPUTF8|SMTPUTF8]], field bodies are restricted to 7-bit ASCII characters. Some non-ASCII values may be represented using MIME [[MIME#Encoded-Word|encoded words]].

====Header fields====
Email header fields can be multi-line, with each line recommended to be no more than 78 characters, although the limit is 998 characters.{{ref RFC|5322}} Header fields defined by RFC 5322 contain only [[US-ASCII]] characters; for encoding characters in other sets, a syntax specified in RFC 2047 may be used.{{ref RFC|2047}} In some examples, the IETF EAI working group defines some standards track extensions,{{ref RFC|6532}}{{ref RFC|6531}} replacing previous experimental extensions so [[UTF-8]] encoded [[Unicode]] characters may be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such addresses are supported by Google and Microsoft products, and promoted by some government agents.<ref>{{Cite news|url=https://economictimes.indiatimes.com/tech/internet/now-get-your-email-address-in-hindi/articleshow/53830034.cms|title=Now, get your email address in Hindi - The Economic Times|newspaper=The Economic Times|access-date=2016-10-17|url-status=live|archive-url=https://web.archive.org/web/20160828044631/https://economictimes.indiatimes.com/tech/internet/now-get-your-email-address-in-hindi/articleshow/53830034.cms|archive-date=2016-08-28}}</ref>

The message header must include at least the following fields:<ref>{{cite IETF|rfc=5322|section=3.6 |title=Internet Message Format|sectionname=Field Definitions |date=October 2008 |access-date=2014-01-09  |editor-first1=P |editor-last1=Resnick  }}</ref><ref>{{cite IETF|rfc=5322|section=3.6.4 |title=Internet Message Format|sectionname=Identification Fields  |date=October 2008 |access-date=2014-01-09  |editor-first1=P |editor-last1=Resnick  }}</ref>
* ''From'': The email address, and, optionally, the name of the author(s). Some email clients are changeable through account settings.
* ''Date'': The local time and date the message was written. Like the ''From:'' field, many email clients fill this in automatically before sending. The recipient's client may display the time in the format and time zone local to them.

RFC 3864 describes registration procedures for message header fields at the [[Internet Assigned Numbers Authority|IANA]]; it provides for [https://www.iana.org/assignments/message-headers/perm-headers.html permanent] and [https://www.iana.org/assignments/message-headers/prov-headers.html provisional] field names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include:{{ref RFC|5064}}

* ''To'': The email address(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below.
* ''Subject'': A brief summary of the topic of the message. [[E-mail subject abbreviations|Certain abbreviations]] are commonly used in the subject, including [[E-mail subject abbreviations|"RE:" and "FW:"]].
* ''Cc'': [[Carbon copy]]; Many email clients mark email in one's inbox differently depending on whether they are in the To: or Cc: list.
* ''Bcc'': [[Blind carbon copy]]; addresses are usually only specified during SMTP delivery, and not usually listed in the message header.
* [[Content-Type]]: Information about how the message is to be displayed, usually a [[MIME]] type.
* ''Precedence'': commonly with values "bulk", "junk", or "list"; used to indicate automated "vacation" or "out of office" responses should not be returned for this mail, e.g. to prevent vacation notices from sent to all other subscribers of a mailing list. [[Sendmail]] uses this field to affect prioritization of queued email, with "Precedence: special-delivery" messages delivered sooner. With modern high-bandwidth networks, delivery priority is less of an issue than it was. [[Microsoft Exchange Server|Microsoft Exchange]] respects a fine-grained automatic response suppression mechanism, the ''X-Auto-Response-Suppress'' field.<ref>Microsoft, Auto Response Suppress, 2010, [https://msdn.microsoft.com/en-us/library/ee219609(v=EXCHG.80).aspx Microsoft reference] {{webarchive|url=https://web.archive.org/web/20110407033540/https://msdn.microsoft.com/en-us/library/ee219609(v=EXCHG.80).aspx |date=2011-04-07 }}, 2010 Sep 22</ref>
* ''[[Message-ID]]'': Also an automatic-generated field to prevent multiple deliveries and for reference in In-Reply-To: (see below).
* ''In-Reply-To'': Message-ID of the message this is a reply to. Used to link related messages together. This field only applies to reply messages.
* ''List-Unsubscribe'': HTTP link to unsubscribe from a mailing list.
* ''References'': Message-ID of the message this is a reply to, and the message-id of the message the previous reply was a reply to, etc.
* ''{{vanchor|Reply-To}}'': Address should be used to reply to the message.
* ''Sender'': Address of the sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.).
* ''Archived-At'': A direct link to the archived form of an individual email message.

The ''To:'' field may be unrelated to the addresses to which the message is delivered. The delivery list is supplied separately to the transport protocol, [[Simple Mail Transfer Protocol|SMTP]], which may be extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter delivered according to the address on the outer envelope. In the same way, the "From:" field may not be the sender. Some mail servers apply [[email authentication]] systems to messages relayed. Data pertaining to the server's activity is also part of the header, as defined below.

SMTP defines the ''trace information'' of a message saved in the header using the following two fields:<ref>{{cite IETF|title=Simple Mail Transfer Protocol|rfc=5321|author=[[John Klensin]]|sectionname=Trace Information|section=4.4| date=October 2008 |publisher=[[Internet Engineering Task Force|IETF]]}}</ref>
* ''Received'': after an SMTP server accepts a message, it inserts this trace record at the top of the header (last to first).
* ''Return-Path'': after the delivery SMTP server makes the ''final delivery'' of a message, it inserts this field at the top of the header.

Other fields added on top of the header by the receiving server may be called ''trace fields''.<ref>{{cite web|url=https://www.ietf.org/mail-archive/web/apps-discuss/current/msg04115.html|title=Trace headers|author=John Levine|date=14 January 2012|work=email message|publisher=[[Internet Engineering Task Force|IETF]]|access-date=16 January 2012|quote=there are many more trace fields than those two|url-status=live|archive-url=https://web.archive.org/web/20120811135249/https://www.ietf.org/mail-archive/web/apps-discuss/current/msg04115.html|archive-date=11 August 2012}}</ref>
* ''Authentication-Results'': after a server verifies authentication, it can save the results in this field for consumption by downstream agents.<ref>This extensible field is defined by {{IETF RFC|7001}}, this also defines an [[Internet Assigned Numbers Authority|IANA]] registry of [https://www.iana.org/assignments/email-auth/ Email Authentication Parameters].</ref>
* ''Received-SPF'': stores results of [[Sender Policy Framework|SPF]] checks in more detail than Authentication-Results.<ref>{{IETF RFC|7208}}.</ref>
* ''DKIM-Signature'': stores results of [[DomainKeys Identified Mail]] (DKIM) decryption to verify the message was not changed after it was sent.{{ref RFC|6376}}
* ''Auto-Submitted'': is used to mark automatic-generated messages.<ref>Defined in {{IETF RFC|3834}}, and updated by {{IETF RFC|5436}}.</ref>
* ''VBR-Info'': claims [[Vouch by Reference|VBR]] whitelisting<ref>{{IETF RFC| 5518}}.</ref>