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
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!