Sendmail Shows Return Address Domain localhost.localdomain
Look in /etc/mail and you'll see a sendmail.mc. Edit this file to the proper domain. You're looking for the LOCAL_DOMAIN directive. Then, you'll have to reconfigure the real sendmail config file (sendmail.cf) by running
make -C /etc/mail
Just a warning- you'll need the sendmail-cf package installed to be able to do this. After that, everything should work pending a restart of the sendmail service.