When you try to create a mailbox by using Exchange Admin Center (EAC) in a Microsoft Exchange Server 2013 environment, the suffix for the User logon name box only lists the root domain names. However, the expected behavior is for root domain names and child domain names to be listed.
This issue occurs because the domain list function only keeps the root domain, and then returns it to the domain picker.
To resolve this issue, install the following cumulative update:
2892464 Description of Cumulative Update 3 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about how to use EAC to create a mailbox, go to the following Microsoft website: