How do I stop my email from sending spam?

My email address keeps sending out spam. Not just to my contacts, but to everyone I've ever emailed, and also back to myself. I've changed my password a number of times, and there is nothing in my "sent" folder. The spam is all from different sources - prescription drugs, cheap marijuana, acai cures, penile enlargement, weight loss... no common theme. Please let me know if there is anything I can do besides changing my email address... Thank you!!!