Why does my outgoing email address default to another unknown address?

Why does my outgoing email address (when typed in the FROM box) change to another email address? It shows my correct address window but when I hit the menu inspector (little triangle) it shows my email being sent by another unknown sender. I believe it is malware or a trojan using my address. How do I fix this? Unfortunately, I can't change my address because it is used by a commercial business. Anybody ever heard of this problem?