Apparent Security Breach

Twice in the last 24 hours, I had outgoing email blocked due to excessive use. I have not sent an email in 4 days. Apparently, this has happened to others as well. Checking my sent mail, mail has been going from my account to unknown addresses containing my login and password in the clear in the both the subject and body.. I suggest all check their sent boxes for bizarrities. I would like the local security gurus to look into this immediately and please keep us all posted.