Unable to Join w2k DOMAIN

I have a NT box to be used as a exchange server 5.5:w/
Sp5
option pack
I tried to join this box to my existing W2k domain. I have a mixed mode w2k domain controller running on the same subnet, but NT tells me that no domain controller is available. I can ping and browse the w2k DC ok. I am using the DC’s ip sddress as a primary DNS on the NT box. the DC is running DNS for active directory. Any ideas why.