If you have ever used your iPhone/iPad to conencto to an enterprise exchange account you will probably of come accross this issue, or atleast your IT person will have.
Since Exchange 2007 there has been a bug where the permissions are not working as they should do, and you have to manually set inheritable permissions on the Exchange servers object in the User's Security settings on their AD Account.
To do this follow the steps below.
Go to your Domina Controller and open up Active Directory Users and Computers.
Click on View and then make sure Advanced Features is ticked as per the image below.
Then scroll down to where your users are located and fine the relevant user, right click on them and then go to properties, and then to the security tab.
Once you are in the click on the Advanced button near the bottom right corner.
Once you have opened it up you need to select Exchange Servers and then put a tick in the "Include Inheritable Permissions" box and then click on OK.
Leave it a few minutes then your emails should start to come through, this is a bug in all versions of Exchange from 2007 upwards and is still yet to be fixed.