Understand Message-ID and Received Headers

Headers tell you which system created the message and how it moved between servers.

Useful basics

  • Message-ID: should exist and be stable per message
  • Date: should reflect a sane send time
  • Received: shows routing hops
  • Reply-To: should be intentional when it differs from From

Warnings usually mean

  • The sending application is generating malformed messages
  • An intermediate relay is rewriting values unexpectedly
  • Multiple systems are involved and you are testing the wrong one

When header warnings appear, inspect the message source and the sending application before changing public DNS.