Me too, I once had a very irate person report me to my isp because of spam coming from my email address. Luckily the address was the one provided by my isp, and not from one of my sites.
The isp tech guy shrugged it off, saying that anyone can spoof an email address by altering the emails header before sending it. My address was probably randomly chosen from a list, although in your case its seems strange that you were also the recipient of the mail at a different one of your addresses.