What Is an Email Header?
TL;DR
An email header is the block of metadata carried at the top of a message, holding routing and identity fields such as From, To, Subject, Date, Message-ID, Received and Authentication-Results. Reading it is how you find out which servers handled a message and whether SPF, DKIM and DMARC passed at the receiver.
Which header fields matter for deliverability?
Authentication-Results first, because the receiving provider writes it and it records the verdict that mattered: the SPF result, the DKIM result with the signing domain, and the DMARC outcome. It is the only field in the message that reflects what the receiver concluded rather than what the sender claimed.
Then the identity and control fields. Return-Path carries the envelope sender that SPF was checked against, Message-ID identifies the message, List-Unsubscribe exposes a machine-readable opt-out, and Feedback-ID groups the message for Gmail complaint reporting.
How do you read an email header?
From the bottom up. Each server that handles a message prepends its own Received line, so the oldest hop is at the bottom and the receiving provider is at the top, and the chain read in that order shows the actual path plus the time spent at each step.
Long gaps between adjacent Received lines are the useful diagnostic. A multi-minute or multi-hour gap is where a message was queued, which usually means greylisting or throttling at the next hop rather than anything wrong with the message itself.
Frequently asked questions
In the client, under an option such as Show original in Gmail or View message source in Outlook. Copy the whole block including the Received lines, since a header pasted from the visible summary omits exactly the fields worth reading.