What are email headers?
Email headers are the lines at the top of every message that record where it came from and how it was checked: From, Return-Path, a Received line for each server it passed through, and the Authentication-Results the receiving server wrote for SPF, DKIM and DMARC. Most mail apps hide them behind an option such as Show original.
Reading the result
Paste the full headers of a message you received. The tool shows From and Return-Path side by side, the SPF, DKIM and DMARC results the receiver recorded, and every hop with the delay at each one. Headers can carry personal data, so they are never cached or logged.
Common problems in email headers
SPF passes and DMARC fails
SPF passed for the Return-Path domain, usually your sending service's own, which does not match your From domain. DMARC needs an aligned pass, so set up a custom return path or aligned DKIM.
DKIM fails on a signed message
Something changed the message after it was signed, such as a mailing list adding a footer or a gateway rewriting links. Or the selector's key in DNS no longer matches the one signing.
No authentication results at all
Headers copied from your own Sent folder never passed a receiver's checks. Send to an outside mailbox and copy the headers from the copy that arrived.
One slow hop
A long delay at a single server usually means a queue there, or greylisting, which defers the first attempt from a sender it does not know.