You are unable to choose the OU in EAC when creating a new Mailbox, Groups or Contacts

Exchange Control Panel (ECP) can display no more than 500 OUs. When there are more than 500 OUs, a new window is generated, and this window is either blank or contains a "There are more items to show in this view" message.

1. On the Exchange Server 2013 (2016 ) mailbox server, go to the following folder:
    
   
   C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp
2. Add the following lines in the web.config file just above </appsettings>:

   
   <add key="GetListDefaultResultSize" value="5000" /> 

3. Restart MSExchangeECPAppPool.