Email Header

An email header is the set of fields before the message body: From, To, Subject, Date, Message-ID, and technical lines added by each server. Most clients hide most of it, but a "show original" or "view source" option reveals the full text.

Why it matters

Headers are how you diagnose delays, spam placement and authentication. Each server that handles a message prepends a Received line, so reading them from bottom to top shows the route and timestamps; a large gap between two lines shows where the delay happened. The Authentication-Results header records the receiver's SPF, DKIM and DMARC verdicts. Anything below the receiving server's own lines can be forged by a sender, so trust only lines added by servers you control.

Example

Authentication-Results: mx.example.net;
  spf=pass smtp.mailfrom=news@sender.test;
  dkim=pass header.d=sender.test;
  dmarc=pass header.from=sender.test
Received: from mail.sender.test (203.0.113.25) by mx.example.net; Mon, 5 Oct 2026 09:14:02 +0000

Common mistakes

• Believing the top Received line proves where mail started; it only shows the last hop.
• Trusting forged lower Received lines.
• Copying headers into a public ticket without removing private addresses.

Related

• From Header
• Envelope Sender
• SPF
• DKIM
• Tool: Email deliverability test

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.