The first thing I do when this happens to me is change my password.
YEP:
http://ask-leo.com/someones_sending_from_my_email_address_how_do_i_stop_them.htmlBefore I get to that, there is always a small possibility that your email account has been compromised.
The solution there is simple: change your password immediately. That should prevent someone who's using your account for malicious purposes from continuing, assuming you've chosen a good password. (If you find that your account has indeed been compromised, you may want to change more than just your password.)
