Three weeks ago We upgraded one of our companies to Exchange 2013 on 2012 R2 from Exchange 2007 on 2003 R2 (64).
In this company there are just a small number of users and after the upgrade only one user could not reconnect using and iPhone6. After waiting two weeks this user gets an iPhone8 and it still will not connect. It would connect to the service yet not sync any mail. After going over the settings on the phone, the exchange box and mimecast we where left with no other option then to reach out to the manufactures. We started with Apple, After 2 hours with Apple support I get to Apple advanced support and am told it is connecting it is just not syncing, to the user this is the same thing and the same conclusion we had started with 3 hours earlier. Apple admitted they were at a loss and if Microsoft had any testing they could help with to please call back. 3 hours and 52 minutes after opening a ticket with Microsoft we get to talk with a tech and once he confirms that the exchange is fully operational and also declares that he does not know, it comes up that in Active Directory when an account has an issue it is most often a permissions issue yet that users account had already been given all available permissions, or so we thought.
In Exchange 2007 on 2003 R2 there is no permission settings in the exchange proprieties.
In exchange 2013 on 2012 R2 there is a permissions setting.
In the entire company only 2 users this setting was not on, and as soon as we turned it on email flowed in to his phone.
Nowhere in all the documentation we read on the blogs searched did anyone even mention this option, and it was just a checkbox.