How to accept but delete all incoming mail

Hello.
I've setup a null client (from the postfix doc) to use a sender rewrite with sender_canonical_maps to send mail from serverX@...  through a mailgateway.

And before finishing of the mailgateway config, I got a bounce back. So how can I fix the null client to accept but drop all bounce/rejects?
The servers were playing ping-pong back and forth with bounce mails :(