Internet Message Format
The standard for the text of an email message, RFC 5322: header fields such as From, To, Date and Subject, then a blank line, then the body.
What it means
RFC 5322 (2008), which replaced RFC 2822 and before it RFC 822 of 1982, defines what an email message looks like as text. A message is a series of header fields, each a name, a colon and a value, then an empty line and the body. Lines end in CRLF and may not exceed 998 characters. Only two fields are required, the origination date (Date) and the originator (From), though every message should have a Message-ID.
RFC 5322 itself covers plain US-ASCII text; other character sets, HTML and attachments come from MIME, layered on top. Receivers enforce the format more strictly than they used to: Google asks senders to follow RFC 5322, to include a valid Message-ID and to use single-instance fields such as From, To, Subject and Date only once, and Yahoo asks for compliance with RFC 5321 and 5322.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322: Internet Message Formatrfc-editor.org
- RFC 5322, section 3.6: Field Definitionsrfc-editor.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Yahoo Sender Hub: Sender best practicessenders.yahooinc.com
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