Exchange Server's DeviceId could be predicted by concatenating "Appl" and serial number for iOS 7 and lower. This technique allows adding rules for a device under iOS even if device is not connected to Exchange Server. But built-in email application generates some new 26-digit DeviceId in iOS 8.
Is there any way to get this new 26-digit iOS 8 DeviceId using serial number, uuid, IMEI, etc.?
Now it's impossible to automatically add a rule for a device by DeviceId beforehand because concatenating "Appl" and serial number does not work anymore.
This isn't a complete answer, but it will give you a start.
In iOS 7, Apple employed a new ID type to register in ActiveSync: EASDeviceIdentifier
This is randomly generated, and seemingly impossible to guess ahead of time. This identifier is the new default ID given to ActiveSync at signup in iOS8.
Source: a blog!
For iOS, I would suggest implementing management profiles and controlling the device more holistically, rather than trying to predict a device ID and enabling EAS for a specific handset. Of course, I know nothing about your environment and couldn't say whether that approach is really appropriate.