Not sure why it would have changed but have you checked your sync settings to be sure your mail app is still set to sync? Also in the app, not certain which one you have, there is often a refresh/sync rate. It could be anything from every 15min to once a day to only when opened. However you didn't say it only syncs when opened so the only other thing I could think of would be have you changed your password? Anything it had before your change would still appear on the phone as though you were looking in your mail, but that content would never update because it no longer has access to your account. Since it is not a GMail account your phone could care less if it syncs or not.
A side from those;
Are you using any kind of power saving, memory booster, or app manager apps? Often these types of apps will see email as a non essential and block them or stop them. Now for my soapbox. If you are using such an app they are not necessary, and most end up causing more grief than help. They stop things from syncing that most people want and have no effect on items that were the cause of them being downloaded. Very rarely do they do more than drain your battery, because they are constantly running in the background, or slow your phone down as they fight with apps for resources to stay on top of things.