Mailer Daemon Demon

Every once in a while I get a MAILER DAEMON notice saying an e-mail address I used couldn't accept whatever it was I was trying to e-mail.
Lately I've been getting about 150 a day, from isps I don't recognize, regarding mail I did not try to send.
Anyone heard of this? Is it a virus or a worm or something?
I have an iMac, I thought Mac users were immune to this kind of thing.
It's frustrating.