I use Mail as my main email program on my MBP. I have 2 accounts (besides the iCloud that I never use): Gmail and an Exchange account that is my work email.

Sometimes in the Exchange account I get the new mail icon (i.e. the little red dot with the number in it and the grey number besides the mail account) but when I click on the Exchange account there are no emails shown. Neither new nor old. It just says No Message Selected in the reading pane. When I restart Mail I am able to read and see the emails. Plus, it seems to do it randomly. I have about 6500 messages in the account.

Any ideas?

Mail 6.5
OSX 10.8.4