Reject, don't bounce. Search the SPAM-L archives for many recent threads as to why bouncing is "bad". A reject will refuse the mail before it is accepted by your system, and therefore does not make finding the correct sender your problem. If you bounce, you have now taken responsibility for sending notice to the correct sender (if you decide to).

It is simple to forge the envelope on a mail message. If you accept then bounce, you can easily be used to send "reflect" spam via your bounce mechanism.

I actually wrote (in perl, just to keep it slightly on topic :) a qmail delayed notification program (still available via qmail web sites, I believe), that sent notices of delayed mail in the queue to the originating party. Now days, I would not consider writing it or using it, except on a set of controlled, internal systems. The qmail "accept then bounce" model is also no longer very good in today's internet. I still use it at home, but I do not bounce, I save for spam filter fodder.