When a spammer sends out their advertisement emails, they are often doing something called email spoofing, which allows them to put any address they want as the sender of the unwanted email. There are multiple reasons why spammers will spoof addresses. One reason is, any mail that doesn't reach the intended destination will be bounced back to your inbox, rather than back to the spammer, who doesn't want to deal with the undelivered mail. A more pressing reason is, the spammer is breaking the law by sending out mass ads in emails, and by spoofing random addresses in the "From" field on these emails, they are attempting to mask their true identity. Unfortunately there currently really isn't much you can do to stop your email address from being spoofed, but you will find that it will eventually stop as the spammer moves on to use someone else's email address. While this activity is illegal, as is spamming in general, it is hard to prosecute an offending party, because they are actively attempting to mask their identity, often succeeding with a combination of spoofed addresses and fake or anonymous email header information. The federal trade commission has an email address set up to forward spam emails to - here is a quote from their website:

"If you get spam email that you think is deceptive, forward it to spam@uce.gov. The FTC uses the spam stored in this database to pursue law enforcement actions against people who send deceptive email."

I'm not sure how helpful the FTC will ever be in stopping spam, but if you are like me and you receive more than seventy spam messages a day, you can help their efforts by forwarding a sample of the spam you receive to spam@uce.gov.