We've always had a 'best together' experience with Outlook, but our core email functionality & many other pieces of Exchange have of course always been open to other clients using standard protocols. In Exchange 2007 we introduced Exchange Web Services (EWS) and in the next version of Exchange (codenamed Exchange 14) we've continued to invest in making EWS even easier to use, with a managed client API, and you can use standard development tools to use EWS to access and manage the email, calendar, contacts & other content in mailboxes.