E-Mail Mail Administrator <Postmaster@cox.net>

I keep receiving emails from  Mail Administrator <Postmaster@cox.net> Subject-Mail System Error - Returned Mail/

When I look at the email, I did not send it.  It has someone else's name then has my email in <my email address  >.  I have changed my password and ran I virus scan but I am still receiving them.    Does anybody know what I should do?