Hi, I Just wanted to understand that why we need to install CAS in 1st place and then HUB and then Mailbox.I know the things can be other way around as well but as per the MS best practice it should go like 1- CAS, 2nd- HUB, 3rd Mailbox. So is there specific reason do we need to install CAS and then others servers. Thanks

Hello Amit In Exchange 2010 the Client Access server role accepts connections to your Exchange 2010 server from different clients like Mapi webaccess or rpc over https. Outlook+ MAPI clients now connect through CAS rather than directly to the mail server. CAS also provides access to directories for any Outlook needs. you must install the Client Access server role in every Exchange organization and every Active Directory site that has the Mailbox server role installed.Thanks Mhussain

Hello In your Scenario i would recommend you to go with 2 Exchange server with a Load balancer you can deploy CAS HUB and MBX in Same server configure CAS Array on Load balancer and DAG Have a look at this article. http://www.msexchange.org/articles_tutorials/exchange-server-2010/high-availability-recovery/load-balancing-exchange-2010-client-access-servers-using-hardware-load-balancer-solution-part1.html Thanks Mhussain

well.. i would say..if this is a clean install of exchange 2010 you not install MB role before HT or client access this is one of the setup requirements .. that is coded in the setup process if you do not have HT server in the same site as your mailbox server setup will fail stating the same as the error. HT is required for pickup emails from users outbox as they compose and does other things like delivering similarly you must have client access for the funcations it performs as stated in earlier comments and if this is updrage you should upgrade client access first as a new version CAS supports older mailbox server however reverse does not work. Dhruv

That's a good question. CAS controls non-MAPI accessibility such as OWA, smartphones, etc. I think Microsoft wants to ensure that users can connect to it before taking care of the back end.

Hello Default OAB is created on MBX server so in this case Virtual Dir should exist before MBX installation so does CAS Hence MBX is dependent on CAS role. Thanks Mhussain