I have certainly heard of such "spoofing," but this is my first personal experience with it. My understanding is that it does not imply that my mail has been intercepted or that my address book was accessed; rather, the address was either found or was randomly generated. MacWorld offers the following advice:
Nearly all of us have had our email addresses spoofed by spammers at one time or another. It’s a regrettable downside of this Internet age in which we live. So, turn your mind from revenge for the moment and concentrate instead on protection and filtering...After about three days of receiving notifications from the Postmaster Daemon, the phenomenon just stopped, without any interventions on my part.
Unless you’re living in the Stone Age you’re aware of spammer spoofing and I’m slightly shocked that there are still Internet Service Providers out there that haven’t done something to filter this junk on their end rather than making it your problem. But, it’s an imperfect world, and because it is I’d suggest creating a rule or two in whatever email client you use that diverts these bounces to a junk folder you create.
So I assume the problem has passed. But if anyone out there has received an email with what looks like my return address in the "from" box, I trust you will understand that I do not sell Viagra and Cialis...