Hello all

I am running Exchange 2013 CU8. I have two AD sites, and two DAG's.  5 mailbox servers in each DAG, and 4 CAS servers in each AD site.  Environment looks similar below.  The picture shows 4 CAS, however i have 8. What i want to know is it best practice to route smtp email through the CAS servers ? what is the advantage to using FrontendProxyEnabled ?

Hi,

In an Exchange Server 2013 organization the Mailbox server role is responsible for sending outbound email via a Send Connector.
When enable Proxy through client access server in send connector, the SMTP connections from the mailbox server to the external world or else to the other exchange organization will be proxy via client access server to the destination server. Those proxy connections from the mailbox servers are received by the outbound proxy front end connector in the client access server.

The Front-End Transport service provides network protection a centralized, load balanced egress/ingress point for the Exchange organization, whether it be POP/IMAP clients, SharePoint, other third-party or custom in-house mail applications, or external SMTP systems.
More details about it, please refer to Transport section in below blog: http://blogs.technet.com/b/exchange/archive/2013/01/25/exchange-2013-client-access-server-role.aspx

Thanks