Sorry, "bounce" is the command for "Resend mail" (not rejecting email) in a lot of unix mailers. Say A sends an email to B who sends it to C.

If B forwards it to C, the email appears to come from B. If B "bounces" it to C, the email appears to come from A.

(it's the Resent-to header in RFC 822 IIRC)

So say you get a bug report from A than you know only C can fix, by bouncing it to them they get to easily reply directly to A and get on with it, leaving you out of it...

I'm not sure about manually bouncing but allowing a filter to bounce an email can let you trick spammers into believing they reached a bad address.