I want to configure my courier daemon to not issue an 550 User Unknown
message during the SMTP transaction and instead fire a mailer-daemon
type email to the sender warning of the problem.
Is it possible at all?
If so, how to do it?
Not normally, but you could set up a courier box as a relay in front of
your real server. It's a horrible idea though. You're going to start
getting a whole LOT of spam and it's all going to bounce into your
postmaster box making it impossible to maintain your server properly.