Comment 3 for bug 317563

It appears to use $USER@$(cat /etc/mailname) in the end, so you might fix it by using your mail hosts' name in /etc/mailname.

Basically, you want to pass the "-f" option to sendmail, so the following might work:
    $CONFIG{'mailer-send'} = [ 'sendmail', '-<email address hidden>' ];

It says however:
    Setting this option is strongly discouraged. Fix your local MTA
      instead.

If I remember correctly, it's easy with e.g. "nullmailer", but does not work as easy with dma (which is the current default MTA in Ubuntu).