Configuring BES to work with 2 exchange servers
 

I have a BES with 10 users connected to an exchange 5.5 server. We recently setup an exchange 2003 server and have started to migrate our users from 5.5 to 2003.

how can I setup the BES to talk to both the 5.5 server and the 2003 server so that when my blackberry users are migrated from one exchange server to the other, that they will have minimal downtime with their handheld?

I've only been able to find white papers and articles that say it is possible, but none of the articles describes how to configure the BES to do this.