Just verify that cdo.dll is the same version on both the BES and the Exchange server. If the BES is lower then copy the newer cdo.dll to the system32 folder and register it. After registering the file you will want to restart the BlackBerry Dispatcher service. This will cause mail delays for a little while depending on how many users you have so you may want to hold off on that. If you have less then 100 users then you should be fine.