What is an email header?

Email headers are lines of metadata at the beginning of every email message. They contain information about the message's journey: who sent it, when, which servers handled it, and whether it passed authentication checks.

Headers divide into two categories:

Standard headers: Defined by RFCs and expected in every message. From, To, Subject, Date, Message-ID, MIME-Version.

Extended headers: Added by servers along the route. Received (one for each server), Authentication-Results, X-headers (custom fields).

Most email clients hide headers by default, showing only From, To, Subject, and Date. The full headers contain far more: the complete path the message traveled, spam scores, authentication verdicts, and server processing notes.

Headers are critical for troubleshooting deliverability. When a message lands in spam or goes missing, headers reveal why. They show if authentication failed, which server flagged it, and where delays occurred.

Headers are the ship's manifest and voyage log combined. They record everything about the journey, even if recipients only see the cargo.