Active Sync with Client Access Server and Exchange 2003 Mailbox Server

When migrating form Exchange 2003 to 2007, at some point of your migration you will have your 2003 mailbox servers being accessed via the new 2007 client access server. At this stage you need to ensure you allow integrated windows authentication on the Microsoft-Server-ActiveSync in IIS or Active Sync will not work. Client Access recieves authentication using basic over SSL, then proxies it to the exchange 2003 backend using windows authentication, thus the box needs to be ticked.