outlook resolve exchange 2000 mailbox

Hi, recently I installed exchange 2000 in our windows 2000 domain. but I met a very strange problem when I resolve the exchange mailbox for most of the user, only few computer succeeded to resolve. It said “The name could not be resolved, the operation failed”, I checked with the DNS and RPING, no problem. I used the IMAP4 and POP3 client to access, no problem. maybe it is a license problem?