Mail delivery failed: returning message to sender

Someone or something has taken over one of my email accounts.   I'm getting 30-40 returned emails a day that I didn't originate with the message as shown in my subject.   Earthlink hasn't been much help and keeps coming back to saying I should run my anti-virus software (NIS 2011).  I've done several complete scans but have found nothing.  I have also changed my email password to something much stronger but that has not helped.  I am attaching one header from a returned email.  Thoughts on how to resolve this?