Basically a spammer (or trojan) uses random addresses at your domain name as the From addresses, and then sends the spam to a load of randomly generated addresses @aol.com, most of which don't exist, so bounce back to you (or have their mailboxes full of "MS updates" so bounce as well).
Although this usually only happens for a week or so, or you can just block bounce messages, or Mailer-Daemon@aol.com etc.
If you're at all sensible you'd have a specific address for registrations and not just use support@ or email@. Unless you're doing your hosting on the cheap, 1Mb of spam a day should not bring a company Email down since Nov. 02!
I worry about copy protection like this too, as what happens when you've corrupted your install or your computer dies, and the company has gone bust without publishing a method to retrieve your files or unlock the software? This is a big problem for relying on closed-source and especially protected software. Mind you, copy-protection is pointless anyway as someone will always find a way.....