Settings>email+accounts. See if any of the accounts are constantly syncing (the account keeps blinking back and forth between white and gray). If so, remove that account and add it back. If it's your MS account, you might have to reset your phone (settings>about>reset your phone). Make sure to back up everything on your phone if this is the case, and it's a good idea to take a screenshot of your homescreen and post it to skydrive so you know how to put everything back the way it was on your homescreen.
This just happened to my wife's 920 - it was the MS account, and resetting fixed it.