I honestly don't think you can. If I were the administrator of the mydomain.com domain which was running Exchange and the MX records were setup correctly, I would receive all the @mydomain.com mail. I would configure users on the mydomain.com domain and give them all a mailbox (
[email protected],
[email protected] etc). It would be a security risk if I were to publish all of the usernames and email addresses on the mydomain.com domain, so I would not do it. You could guess that if Fred Bloggs worked on the mydomain.com domain his email would probably be
[email protected] or
[email protected] but that is as close as you will get. Here endeth the lesson.