We got a lot of mail to unknown recipients - almost 100% spam.
And now we are on a blacklist due to the return messages (550 No such recipient (in reply to RCPT TO command)).

Is there a way to disable the return message to the sender when the recipient isn't known in the system?

I really can't see the use for it. When an employee leaves, we initiate auto-reply for a period, and after that we are happy to just delete future mails to this address.

/Jens Jakob Sørensen
Dansk Centralbibliotek