One ADC 2 windows 2000 advanced server

I got 3 Windows 200 advanced server: ServerA, ServerB, ServerC. Server A act as primary server meant that all user list ADC in ServerA, ServerB as mail server meant that MS Exchange 2000 server had be install.
How to use server A user list in server B mailbox?
Is it I have to setting a connection agreement in both server A and B?? Please advise me ! Thank you