I run secondary mail servers for a bunch of people, so my queue is constantly full of bounces since both spam and viruses seem to send to the secondaries as a matter of course now :( By the time the primary has rejected the mail at the SMTP level, the secondary has disconnected from the sender, so it has to send a bounce message.
I have been steadily disabling secondary MXes for some time now. Its just a great way to attract spam - not an option when you net connection is not stable/permanent, but aside from that secondary MX just seems to be spam trap.
dave -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
