1. Install "Network Load Balancing" feature on each CAS+HT servers.
2. Setup the NLB (in my case my nlb name is "webmail.demo.local"), exf1 and exf2 are the members, also add it to DNS.
3. set a new client access array using new-ClientAccessArray command, it will be used among outlook clients (MAPI) to connect to the exchagne 2010 servers.