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What do X-headers in mails stand for?

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I'm wondering what all X- headers in e-mails stand for.

  • Are they really just a custom and everyone can make them up?
  • Are they documented somewhere? I've quickly browsed RFC 5322. It does not seem to mention anything about custom headers.
  • When or by whom were they introduced?

Thank you for shedding some light in that field :)

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