Sounds like your email acct has been hacked/hijacked. First do a scan with a good antivirus & spyware program(s). Then after it's done & hopefully found & deleted something for you, change your email password. If you just try changing your password & there's still a keylogger or the like on your computer they'll just get your new password anyway so make sure it's clean first.

You'll still get delivery failure messages for emails already sent but it shouldn't send any new ones after the above.