Cannot connect to a exchange server

This looks like a client related problem.

I have 2 exchange servers
After a user login successfully on a client (xp sp1) he cannot connect to exchange server A, but the user connects successfully to exchange server B.

No other user can connect to exchange server A on this client.

If the same user try to connect via another client on exchange server A, no problem.

From 500 clients, 50 have this problem. The rest can connect successfully.
It looks like this problem is spreading.

Who have a solution for this problem?

Thanks in advance
Gr, salomon