Hello,

Does anyone know how to setup a default domain name with Exchange 2013?  Here's an article on how to do it in Exchange 2010: ActiveSync default domain with Exchange 2010.

We need to do this because most users have their iPhone's connected to an existing server that defaulted their domain name.  So their trust relationship and security settings are authenticating without the need for a domain name.  A few users that we test migrated over to Exchange 2013 immediately failed to authenticate because of this reason.  As soon as we went into the iPhone configuration and added the domain name it started working.

Have you tried making their user principal name the same as their e-mail address?

Please go through below articles, will help you resolve the issue:

http://technet.microsoft.com/en-us/library/bb123679%28v=exchg.150%29.aspx

Exactly what in that article helps with the posted question?