If you have more than 500 Organization Units in your AD forest, you may..no…you will, run into this issue in the Exchange 2013 EAC when creating a new mailbox or group and want to create the object in a different OU other than the default “Users” container.
Upon accessing the OU Dialog box:
You will see this lovely message:
Unfortunately, this is a known issue. There is no fix yet.
The work-around for EAC:
P.S. If you do not know how many OUs your forest has:
Get-OrganizationalUnit -ResultSize unlimited | Measure-Object