If you are using mailx (sendmail) then the sender is automatically input. As for a mail server you don't need one. Each sendmail client is capable of being its own mail server. You can identify this to your Notes machine with an MX record in DNS. That might be hard to do or very easy depending on your network setup. If the NT machine and the HP-UX machine are on the same DNS domain then the MX record is probably already there (unelss the NT machine identifies a separate domain as the maildomain). In that case you may need to add the MX record to your DNS domain. If the HPUX machine is on an internal only network and the NT machine does not belong to that DNS domain then you have a problem, because you can't advertise the MX recordanywhere that the NT machine will find it. The alternative is to have the Notes mail server accept the HPUX domain as a valid mail domain within the Notes configuration and then point the HPUX machine to the NT machine as its mail server. You can do this in /etc/mail/mailserv.
Good luck.