As CS_DNA says:-
Definitely do not bounce... 
The vast majority of spam now comes with forged sender addresses, of both the entirely fictitious and perfectly legitimate type. The latter being a mix of addresses harvested from various places, and addresses either guessed as being legitimate or guessed at random and which turn out to be legitimate. Either way, it's only a miniscule proportion for which a bounce would go to the right place.
So all bounces do is (a) clog up everybody's systems with more junk that people don't wan't; and (b) worry and irritate the recipient, if it's forged
and happens to have a legitimate address....
So what to do...
Well [now that you've got me started on one of my pet peeves
]
Most of the so-called "solutions" that we are seeing today are more or less sophisticated versions of "hitting the delete key". Doing this, in howerver a sophisticated fashion, provides
zero, absolutely zero incentive for either the spammer to stop or the people who are facilitating him (inadvertantly sometimes, for sure) to stop providing him with services.
I contend that:
Until the cost of sending spam, in terms of inconvenience as well as financially, starts being passed back to the spammer (or at least his ISP or other faciltators), nothing will improve and indeed the situation will get worse.
Well there
are techniques which are available which
do actually start passing the cost
back towards the spammmer (or those who are aiding and abetting him.) The problem with them is that they involve a certain amount of short-term pain, for longer term gain -- which is why few people deploy them. The "auto delete" functionality that we mostly see is the opposite extreme: long-term pain (there is
no incentive for the spammer to stop) versus short term gain (the spam
seems to go away.) It doesn't go away, of course, it just
seems to and that's why this approach is just storing up trouble for the future.
Well, enough of that rant for now, if anybody is actually interested in "Plan B" post a message and I'll followup with a little more detail. Off course, as in a lot of things, this issue tends to gnerate a lot of heat as well as light
with factions polarising at opposite ends of the spectrum -- although I have fairly striongs views (you never guessed, did you) my main concern is that people make
informed decisions, rather than ill-informed knee-jerk reactions that I'm seeing from some people recently on this issue (not on Prune, I hasten to add
)