"The name could not be matched to a name in the address list"

I've been trying to join an Outlook XP and Outlook 2000 client from different
Win2k Pro PC but I keep on getting this error ""The action could not be
completed. The name could not be matched to a name in the address list"

I have checked on the Advance Tab on the recipient properties and made sure
that the Hide from address book is unchecked. Network connections seems to be
OK Other Outlook clients is operating well except for configuring new Outlook
clients to join the Exchange 5.5 SP4 server. It's running on Windows 2000
Server SP4. Other done that, I am not sure where else to checked.

Thanks in advance for helping!
spark