Our Exchange organization have 10 thousands users, and about 65% are online at the same time. Two CAS server Two Hub server (Hardware load balance) Two Mailbox server (windows NLB) Symantec mail gateway. I think Exchange 2007 CCR is not designed for so many mailboxes, but how many users it designed for. I want to prove it not suibable to use Exchange 2007 CCR to support so many users. Could you please give some related links to prove it. Any other suggestion is preferred, include update to Exchange 2010 DAG. I hope somebody can give some links to prove we need to update to Exchange 2010.

Did you check Exchange Capacity Planner?Where Technology Meets Talent

The capacity is not the problem. I have enough capacity.

Well, depends on the usage and profile of the users. but I do believe CCR can handle more than 10000mbx - http://technet.microsoft.com/en-us/exchange/bb412164 Some solutions tested. 65% is 6500 users. This can easily be achived with a CCR cluster if this is the usage. However may not be ideal but can work. Maybe create another CCR cluster and further split the users. Moving to 2010 is a good choice and has benefits. If you're looking for a business case to move to 2010, then it's going to take some time to put it together and you have to show the benefits both technical and for the business.Sukh

Thank you Sukh. From the link i know Exchange 2007 can support more than 10000 users. I am thinking about deploying the second CCR cluster.