On BlackBerry Enterprise Server 5 the dispatcher uses an algorithm to distribute users over message agents.
There are two algorithms available, simple and complex. By default the complex algorithm, which groups users based on their exchange server, is used. The simple algorithm just balances users equally across all agents.
In a pure Exchange 2007 or 2010 (and higher) environment where all connections to the mailbox will go through the Exchange CAS (Client Access Server) all users will seem to be on the same Exchange server. In this situation the complex algorithm delivers a sub-optimal result.
To return to an optimal configuration you should switch to the simple user distribution algorithm. This is performed by a simple registry edit on the BES.
Start regedit and browse to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Research In Motion\BlackBerry Enterprise Server\Dispatcher.
I presume a 64bit version of Windows Server is used, on 32bit machines omit WOW6432Node from the path.
Create a DWORD value named ‘AgentOptimizeAlgorithm’ and set the value data to “1” to use the simple algorithm. Removing the value or changing the value data to “2” will enable the complex algorithm again.
After the registry edit the dispatcher must be restarted to make the change effective.
If you are using BES in HA (High Availability) mode you need to perform this edit on both nodes in the cluster.