What you are trying to do has nothing with SMTP to do. It's fully MS Outlook specific. You would have to write a mailhost that does not use SMTP, but integrated directly to an outlook server. I have no idea of how to do that, or even if it's possible. What you might want to do instead, is too see if you can make the fax software you are using be a SMTP-mail to fax gateway. I.e., if you somehow can send SMTP mails to it. In that case, the mail adress would be something like "555-28376@fax.your.server", which would be a valid SMTP address.