A single user cannot connect their iPhone to Exchange with ActiveSync. The partnership appears to be created as contacts and calendars are synced OK but mail is not (although mail folders are...) the event logs are showing "An exception occurred and was handled by Exchange ActiveSync. This may have been caused by an outdated or corrupted Exchange ActiveSync device partnership. This can occur if a user tries to modify the same item from multiple computers. If this is the case, Exchange ActiveSync will re-create the partnership with the device. Items will be updated at the next synchronization."
The first thing I tried was to remove the device using the ECP. This made no difference.
I then ensured the "Allow inheritable permissions from the parent to propagate to this object and all child object. Include these with entries explicitly defined here." was selected and clicked the Default button to reset the permissions.
Using the same logic I did in this post http://blog.ryanbetts.co.uk/2014/10/exchange-2010-sp3-event-id-1053.html I decided to try and give the Exchange Servers principal Full Control access to the object.
I re synced the iPhone and mail started to flow again.