The email header is the block of information at the top of every message that most email apps hide. It records who sent the email, the path it took between servers, and how the receiver checked it.
What's in it
- Received: one line per server the email passed through, newest first, with times. Gaps show where it was delayed.
- Authentication-Results: the receiver's verdict on SPF, DKIM and DMARC.
- Return-Path: the envelope sender, where bounces go.
- List-Unsubscribe: the one-click unsubscribe address, if there is one.
Why it's useful
When an email lands in spam, the header usually says why: a failed DKIM signature, SPF that didn't align, or a long delay at one server. In Gmail, open the message, choose Show original, and paste the text into the free email header analyzer.