To send e-mail to any other Internet mail-based system, use the Exchange 2000/2003 SMTP Connector. As it sounds like the HP Unix system is local, you could set up an SMTP Connector for the specific Address Space (domain.com). In all likelyhood, there is already an SMTP Connector for the Internet with an Address Space of "*". By setting up the specic Address Space, Exchange will know to not send the mail for your HP Unix users out to the Internet. It would be necessary to send e-mail to the fully qualified address of firstname.lastname@example.org. The administrator could create Active Directory Contacts for each of the users on the HP Unix system so that Exchange users would see the AD Contacts in their Global Address List.
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