Há um ótimo podcast de vídeo que explica em detalhes as etapas que você deve seguir em seu servidor e no iPhone para permitir o acesso ao Exchange.
TI Idiotas - Episódio 69: Exchange, ActiveSync e o iPhone
The iPhone is out and it now has a place in the corporate world. In this episode we discuss the Exchange ActiveSync features such as the new autodiscover, remote wipe and enforceable security policies and then take you step by step through the configuration of Exchange 2007, DNS and the iPhone itself. Oh yeah and this feature is provided out of the box, no BlackBerry licenses required!
Em seguida, leia o documento em PDF da Apple, que também explica os detalhes sobre como configurar seu servidor Exchange para o iPhone:
iPhone e Microsoft Exchange Server (PDF)
iPhone 2.0 software communicates directly with your Microsoft Exchange Server via Microsoft Exchange ActiveSync, giving users push email, contacts, and calendar. Exchange ActiveSync maintains a connection between Exchange Server and iPhone so when a new email message or meeting invitation arrives, iPhone is instantly updated.
If your company currently supports Exchange ActiveSync on Exchange Server 2003 or 2007, you already have the necessary services in place to support iPhone 2.0 software—no additional configuration is required. If you have Exchange Server but your company is new to Exchange ActiveSync, review the following steps to enable Exchange ActiveSync.