In this article the msexchangeteam.com has posted how CAS proxying works with ActiveSync.
There are three steps involved in handling a request:
- Determining whether to proxy a given request
- Proxying the request
- Causing the proxied request to execute on behalf of the user that issues the initial request
The mobile device communicates with the First CAS via AirSync protocol. When a request is received by the Internet-facing Client Access Server, a decision must be made whether to handle it directly, proxy it, or return an error. The proxy decision relies on the Microsoft Exchange Active Directory Topology Service (MSExchangeADTopology) to determine if a request must be proxied.
The following logic will be used for each request that is handled by an Exchange 2007
Read more at source: http://msexchangeteam.com/archive/2007/10/12/447266.aspx