If you find yourself setup up a reverse proxy for Microsoft Exchange 2007 using Nginx you may be using this article for your conf http://planetit.ws/linux/debian/configuring-nginx-based-reverse-proxy-for-owa.html
While that article is good it is missing one line from the conf that will allow all mobilbe devices but iPhones/iPads to verify account information using the Microsoft Activesync protocol.
proxy_set_header Accept-Encoding "";