The answer to your question of whether you can combine all servers into one BlackBerry domain will depend on the ping times. The ping times between the BES and mail servers and SQL server is very important. It's highly recommended that the ping between the BES and mail server is less than 30ms. Higher ping times will cause timeouts and in Exchange environments, cause a higher percentage of UDP packets to be lost (users will complain of getting messages in 15-30 minute intervals). With high latency, you also will see more hung threads on Messaging Agents. Ping between BES and SQL should be under 350ms. If SQL is installed on Windows Server 2003, you should install the database notification system (look in install guide for instructions).


The BESAdmin account must be online and unlocked in order for the BES to access users' mailboxes in Exchange environments. Depending on who you talk to at RIM, they may recommend that you use different BESAdmin accounts for each BES or HA pair (primary/standby) in Exchange environments whether they're in the same BlackBerry domain or not. The real purpose for this is because of connection limits with Windows 2008 server based Active Directory. For myself, I just use the same BESAdmin account...it's just easier. Just keep in mind that the BESAdmin account has access to all users' mailboxes (assuming permissions are set on all Exchange servers). The more BESAdmin accounts you have, the more accounts that have access to all mailboxes.


For the Administrator with 4 BES servers and 9000 users...you're a brave man. RIM doesn't recommend more than 2000 users per server but I personally only go up to 1200 users per server. I just see too many problems going above that number. Use the BlackBerry Enterprise Transporter in the 5.0.x resource kit to move users between BlackBerry domains. With 4.1.x as the destination, you may run into the User ID issue though.