I've set up some specific rejection messages for certain recipients, but when I test them, the SMTP message is always 550 5.1.1 <[[email protected]](mailto:[email protected])>... User unknown.

That is not an Exim message (the

5.1.1..

is a clue; Exim doesn't use those extended codes). You are probably being defeated by software that sees the 550 error code, and insists on putting in its own text. There is stupid software that does this. You can test Exim by using -bh or making a telnet call to the SMTP port. That way, there's no other software intervening.

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