Your email id and password have been compromised. Change the password to a SECURE password.
If it keeps up - you probably have a virus and/or keystroke monitor.
And Mac's CAN, and DO get viruses:
http://latimesblogs.latimes.com/technology/2010/04/mac-windows-security.html
EDIT: Had to go find it, knew it was out there... here's a Google search which bring back over 60,000 hits about one NEW virus on Mac's:
http://www.google.com/search?q=HellRTS
Hear it directly from Apple:
http://lists.apple.com/archives/augd/2010/Apr/msg00034.html
Been to any good Torrent sites, lately?
http://www.intego.com/news/ism0901.asp
Have ya had a problem watching a video, lately:
http://www.f-secure.com/v-descs/trojan_osx_dnschanger.shtml
Have you spell checked anything, lately:
http://www.itworld.com/security/102848/mac-os-xs-five-biggest-security-fixes
Oops. Kind of puts a hole in the "I'm on a MAC so it can't be viruses" thing, don't it?