If you haven’t yet updated your iPhone/iPad to the new 6.1 or 6.1.1 updates good ! Apple are working on a fix for an issue with syncing calendars with exchange servers that can cause the server to slow or stop responding. This can then have a knock on effect of stopping email working for other users connected to that server. If you have updated then please switch off calendar sync on your iPhone/iPad this is under the accounts section of the settings app. Apple have tried to fix this issue once already and failed but report they have another update coming soon.
Apple have just released update 6.1.2 for iOS this should sort the exchange bug but not the passcode bug that can allow someone to by pass your screen lock so look out for another update for your iPhone soon