This might just be net-lore, but several of my friends vouch for it's accuracy...

Apparently there are some companies that collect e-mail addresses that are returned to them with 'reject/remove' messages. They don't resend to them, but do sell them to other companies! Since I've learned this I just hit 'delete' rather than sending a 'remove' response. It might be coincidence, but my spam receipt is down slightly.

Note: I do get spam on all my accounts to some extent, but the main one that gets it is one not posted here (my WORK account, arrrrgh!).