First of all try clearing the cache and defaults for the email app on your phone. If that doesn't work, re-install the ROM after a wipe. (This assumes your exchange server address and credentials were all entered correctly)
Also, it could be a DNS error. I run into this every once in a while. If you are connected on Wi-Fi to the local network that the Exchange server is on, you might have to turn off Wi-Fi and try entering the server information again. I sometimes have this problem with certain Android handsets, it can be due to local network configuration not updating the phone's DNS address while on Wi-Fi.