Hi, we are transitioning to Ex2k7. There's one AD site that spans two physicallocations connected by a reliable high-speed connection. There are DCs and GCs in both locations. We plan to place a Mailbox/HT server in each physicallocation (about 200 users in each). Any issues with placing just one CAS server in one of the locations? What's the best practice in this situation?

Hello, Let me ask you a question, what happens if link goes down or disaster happen for CAS server which is at different location? I hope you would have understood my suggestion as well as recommendation from Microsoft that at least a CAS server in each AD site (in your case AD site extended to two different location but that might not be ideal for each case). So recommendation goes to implement at least a CAS server at both locations. Since you are going to implement Mailbox & HT server at both locations you can combine CAS Server with any of these server also if you dont want to give separate hardware for it. You will have only 200 mailboxes on Mailbox server so it will not generate really heavy load, you may calculate the recommendation of Mailbox Server load with Mailbox Server Role Storage Requirement Calculator too. Reference: Exchange 2007 Mailbox Server Role Storage Requirements Calculator updated to v 16.0 http://msexchangeteam.com/archive/2008/09/23/449899.aspx