You should setup you exchange server to use a smart host to send the outgoing emails.
Check the following Microsoft Knowledge Base article for information on how to configure an SMTP connector:
http://support.microsoft.com/kb/265293You should set POP3.mydomain.com as your Smart Host in the SMTP connector, and set the connector to route all email (*) through this server. Normally you can use any account to authenticate regardless of the real address sending the message.
Is this what you wanted to do or do you have a different issue?
Regards,
Eclarsys Support