iPhone 4S works great and syncs with my Exchange email account everywhere... except when I'm in my office's internal WiFi network. It won't send or receive emails from Exchange. Everything else (internet, gmail etc.) works fine and the phone syncs up fine when I go back to 3G or different WiFi network.

The only thing I can think of is that on the office WiFi the iPhone is getting the internal IP address which is different from the external IP and this is causing the problem.

I know it's a long shot, but anyone have the same issue/found a solution?

 

Certainly a configuration issue with the wifi network in question. If you're getting a private IP on the phone and using NAT, and the mail server is on that same network, it's possible whatever device that is doing NAT isn't configured to properly forward packets to another server on the same network.

Could also potentially be a configuration issue on the exchange server if you're talking to it via a network it doesn't expect access from.

 

The server address you are using is being treated differently on the local network.

A quick solution is to turn off WiFi completely or you could leave WiFi on and tap "Forget Network" of your office network so your iPhone doesn't connect when you walk on premsis and loose the ability to connect..