Bounceback static
I get a sizeable number of messages every day from e-mail servers that intercept virus-ridden e-mail with my address in the header. Now, I know I'm not infected, because I'm using Mac OS X -- the offending messages have spoofed return addresses from PC users' address books.
It's a pain in the ass. If I was a system administrator at a company using one of these products and it were up to me, I'd likely turn them off, because I can't see how the bouncebacks really help matters much: They're not directing the messages to the actual infected user, and chances are good that the infected user probably wouldn't know what to do even if they were advised they were infected.
I'm starting to wonder at how much volume these automated messages add to daily traffic across the Internet -- the total volume has to be mind-boggling. Any way you slice it, the amount of work we have to do each day just to rid ourselves of irrelevant crap is truly depressing.