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I'm trying to understand the reality of system e-mail messages. I am currently running an outgoing MTA (Postfix) instance, and there is a couple of details I cannot manage to put my finger on.

First of all; I was expecting system messages to come in at the address that I used as envelope sender for the message that caused the system message. This expectation turned out correct most of the time, but once in a while I notice system messages coming in at addresses coming from the payload's From, Reply-To or Sender headers. How about that? Is it just crazy MTAs sending those out? Or is there a rationale behind it?

Secondly; which types of system messages can be distinguished? I'd say we've got:

  • hard bounce (sent out by an MTA when it cannot pass a message on to the next MTA as this one refuses to accept it);
  • soft bounce (sent out by an MTA when it has already accepted the message, but then cannot deliver a message to the requested mailbox, for whatever reason);
  • auto-reply (sent out by an MTA at the request of one of its users).
  • ...?

Am I missing something? Misunderstanding something? Overcomplicating something? Please shoot at me! Thanks!

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1 Answer 1

Normally a bounce should have an envelope sender of <> and an envelope recipient taken from the envelope sender of the original message. Everything should do what you call "hard bounce"; Exchange doesn't and will accept messages and then generate a bounce. This is bad as it causes spam backscatter.

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