I am having the same issue as post http://social.technet.microsoft.com/Forums/exchange/en-US/7401392f-a822-4a97-8a8b-a17adf16549e/ecp-not-retrieving-all-my-ous-when-creating-new-mailboxes?forum=exchangesvradmin
The solution cited there does not work for us.
By editing the web.config file on the server which resides in C: \ Program Files \ Microsoft \ Exchange Server \ V15 \ ClientAccess \ ecp \
add the following under the appsettings section of the file.
<add key="GetListDefaultResultSize" value="1000" />
We are on exchange 2013 CU3 Windows 2012 Mailbox and Windows 2008R2 for the CAS boxes.
I have changed the web.config file for the ecp as stated above, starting with 1000 then 2500, 5000, and even 200 but no change in the number of OUs displayed.
I've just counted and ECP displays 500 OUs even though I've set web config higher than that. Is there someone who can say that this is a hard set limit? I have also set it 200 and the ECP still displays 500 OUs.
