During the Microsoft Tech-ED date Nov 6, 2009, a lecturer confused me in the understanding of the CAS role in a complex single domain environments. He said that the Client Access server would take care about all the mailbox servers in the domain no matter the mailbox server deployed in the same AD site as the Client Access server or not. He is very certain about this however I found it conflict with what it is described on Exchange Server Library. In order to found how the CAS works, I deside to do the below experiment.
The AD sites and services are setted as the picture shown. A window domain named Exchange.com is created with two AD sites, EX-AD and EX14-AD-site2. The two AD sites located in two subnets, and EX14-AD-site2 has an additional NIC adapter to connect to EX-AD.
In subnet, I installed two Clients Access servers, EX-CAS and EX-CAS2, a mailbox server, EX-MBX and a Hub Transaction role server, EX-Hub. In sub net, I only installed a mailbox server, EX-MBX-site2.
Then I created a user01 on EX-AD with a mailbox located on EX-MBX. After that, I site the customer maschine IP, and logon as user "user01", trying to send a mail via Outlook2007 theneverything is ok. Next, I created an other user "user05" on EX-AD-site2 with a mailbox located on EX-MBX-site2, then set the customer maschine IP and logon as the "user05".  An error occured after I installed the MS Outlook2007, it said that it cannot logon exchange server.
The conclusion is that, each AD site that contained a mailbox server mast have a corresponding Client Access server installed.