connect exchange 5.5 and exchange 2k

Before upgrade, Site A and B are in the same organization. Site A is 5.5 built on win2K AD, Site B connect to Site A through RAS connector. After upgrade, Site A is exchange2k, and site B is still 5.5 without any change. Site A can’t connect with site B after I built X.400 or SMTP connector between Site A and site B. The messgaes and directory service files can’t get replicated between them.
who can tell me how to connect such A and B?