Personally I would recommend that automatic replies show the original sender's e-mail address instead of Postmaster or Mailer-Daemon.
To enable this feature, should you wish to, in the GroupWise Admin Console browse to and click the GWIA. On the SMTP/Mime tab click on Address Handling. Under Outbound Settings place a tick in the box next to Use GroupWise User Address as Mail From: for Rule Generated Messages.
Please let us know if this solves your problem or if you specifically want rule generated messages going out form Mailer-Daemon/Postmaster.
You can't, and by the RFC aren't allowed to. As Laura said, however you can configure your system to send from the original account email instead. But mailer-daemon is used in other places too, like automatic error messages, and it's required "by law" to show the sending FQDN hostname.