Dave M. MacOSG Founder/Ambassador An Apple User Group iTunes: MacOSG Podcast Creator of 'Mac611 - Mobile Mac Support' (designed exclusively for an iPhone/iPod touch)
Here's a good link on the topic. You more than likely don't have anything on your Mac doing this. Chances are your email address was hijacked and spammers are sending mail using your information. This link has a number of suggestions to help out: