Understanding Exchange ActiveSync coexistence
So that Exchange ActiveSync can function during Exchange 2003 and Exchange 2010 coexistence, you'll need to configure Integrated Windows authentication on the Microsoft-Server-ActiveSync virtual directory on the Exchange 2003 server.
When you have some user mailboxes on Exchange 2003 and some user mailboxes on Exchange 2010, the two versions of Exchange Server are coexisting. The Exchange 2010 Client Access server will proxy connections to Exchange 2003 for users with mailboxes on Exchange 2003. In order for this proxying to occur, the authentication method on the Microsoft-Server-ActiveSync virtual directory must be set to Integrated Windows authentication.