On a new Exchange 2013 CU8 environment that I am currently working on, we experienced an issue with the Exchange Admin Center (EAC): when creating a new mailbox for a new user and trying to select the OU where the user should be create, we were unable to see some OUs.
It turns out that, by default, the EAC only displays 500 OUs to optimize EAC’s performance. However, in this environment there are over 900...
The solution was to edit the web.config file for the ECP. On a CAS server, this is located by default at C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\ecp, while on a Mailbox or multi-role server it is at
C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp.
Once you locate the file, open it in Notepad with Admin rights, locate the <appsettings> section and add the following key:
<add key=”GetListDefaultResultSize” value=”1000” />
Obviously, if you have more than 1000 OUs, simply increase the value.
When this is done, go to IIS and recycle the MSExchangeECPAppPool application pool. Reopen the EAC and you will now be able to see all OUs in your organization. Just do not forget to change this setting across all your servers otherwise the behaviour might be inconsistent.