Glossary

Header / Email header

Header / Email header

An email header is a structured block of metadata that precedes the body of an email and contains essential routing and authentication information. Headers are generated by each mail server the message passes through, creating a trail from sender to recipient. 

Common fields in an email header include: 

  • FromToSubject, and Date (user-visible metadata) 
  • Return-Path / Envelope From 
  • Message-ID 
  • Received lines showing the server hops and timestamps 
  • Authentication-Results showing SPF, DKIM, and DMARC checks 
  • MIME-Version, content type, and encoding details 

Email headers are primarily used by: 

  • Inbox providers, to assess authenticity and filter spam 
  • Postmasters and senders, to debug delivery issues 
  • Email security tools, to trace spoofing, phishing, or misconfiguration 

 

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