Email header
The block of header fields at the top of an email message, holding sender, recipients, date, subject, routing trace and authentication results, separated from the body by a blank line.
What it means
RFC 5322 calls it the header section. Some fields are written by the author's program (From, To, Subject, Date, Message-ID), some by servers on the way (Received, Return-Path, DKIM-Signature, Authentication-Results), and some by sending platforms for their own purposes (List-Unsubscribe, Feedback-ID, X- fields). Mail programs show only a few; in Gmail the whole header is behind More, then Show original.
The header is the first place to look when diagnosing delivery. Received lines show each hop and its timing, Authentication-Results shows how SPF, DKIM and DMARC were evaluated, and receivers add their own verdicts, such as the spam confidence level (SCL) Microsoft writes into X-Forefront-Antispam-Report. Gmail also limits header size and rejects messages whose headers are too large.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 2.2: Header Fieldsrfc-editor.org
- Google, Gmail Help: Trace an email with its full headersupport.google.com
- Microsoft Learn: Anti-spam message headers in Microsoft 365learn.microsoft.com
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
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