NT Domaincontroller kann nich gefunden werden

I have a 5.5 server that when you select a mailbox, and click
properties, it will crank for 10 or so seconds and return the message
"Failed to contact Windows NT Domain Controller". I cannot display any
permissions.

If I click OK, the message returns, if I click it again, Exch will
display the mailbox properties. Unfortunately, the Primary Windows NT
Account shows \Unknown Account, and the same error occurs if I try to
change it. Strange thing is, I can browse the user names, it just will
not let me designate one.

AD is installed, Replication is fine.

DNS resolves fine, WINS is running.