I successfully installed a Blackberry server with a Exchange 2007 server but now one of the users in a branch office(Exchange 200) wants to be added to the Blackberry server. When he tries to activate he receives the Server is not responding error. Do I have to modify the rights on the Exchange2000 Server ?
I added the BESAdmin to the user's security to SendAs. They will be getting their own Exchange 2007 server by year end.

First thing to check is if the person's device has had the data plan provisioned properly. Do they have a BES data plan?

another one that got me back in the day was... does the user have any junkmail / antispam filtering going on? if the initial email the handheld sends out get moved out of the inbox, you are going to get that error as well.

You need to make sure the BESAdmin account also has permissions on the Exchange 2000 server. Are both in the same Admin Group?

What rights should the BesAdmin have on the Exchange 2000 server?