The word for this when it is done in excess is a "Joe Job", which as the story goes is named after "Joe" who lost his online business because spammers forged his addy as their return address and he was bombarded by complaints, resulting in the ISP pulling the plug (unjustly) on him.
Moral of the story is, if you start getting dozens and hundreds of these a day, call your ISP, let them know there is a Joe Job going on, and ask their help in filtering or defending against it. A few here and there are next to impossible to prevent short of closing the mail account.
Thanks
Trevor
CanadaRAM.com