Email headers

What shows up in an inbox, subject line, sender name, body text, is only part of a message. Every email also carries a set of headers a person never sees by default: fields like From, Return-Path, and a Received trail recording every server the message passed through on its way. One header matters more than most for deliverability work: Authentication-Results, where a receiving server writes down whether SPF, DKIM, and DMARC actually passed or failed for that specific message.

Why a header check beats a 'looks configured' glance

Entering an SPF or DKIM record correctly in DNS is not the same as confirming it works. The only way to know a message actually passed authentication is to open the raw headers of a real test send and read the Authentication-Results line directly, rather than trusting that the DNS setup looks right. A typo in a record, a stale cached lookup, or a mismatched signing key only shows up once someone reads that header on an actual message.

One more header worth knowing by name: List-Unsubscribe. Google and Yahoo's bulk-sender rules require a working one-click unsubscribe option on qualifying mail, and that requirement is implemented as a header, not a visible link in the body. A message can look fully compliant to a reader and still be missing the header that actually satisfies the rule.