Microsoft Exchange – OU picker is empty when creating new user

We had a case where creating a new user from ECP on Exchange 2013 worked right up until the moment the admin needed to choose the target Organizational Unit (OU). The OU picker opened, but it was effectively empty, which made the mailbox creation workflow a lot less useful than it should have been.

What the issue looks like

The OU picker was not displaying any OUs at all.

That immediately suggested the problem was not Active Directory visibility in general, so a quick check in Exchange Management Shell was the next step.

If Get-OrganizationalUnit returns data correctly, the problem is often the ECP result limit, not missing OUs in AD.

Fix

This is a known Exchange behavior. Exchange Control Panel only handles a limited number of OUs by default, so environments with larger OU structures can end up with a blank or incomplete picker.

First, count the OUs:

(Get-OrganizationalUnit -ResultSize Unlimited).Count

Then modify each server's web.config:

C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\web.config

Add the following just before </appSettings>:

<!-- allows the OU picker when placing a new mailbox in its designated organizational unit to retrieve all OUs -->
<add key="GetListDefaultResultSize" value="2000" />

Use a value that is higher than your OU count.

After that, open IIS, find MSExchangeECPAppPool, and recycle it.

Do this on every server that hosts the Mailbox role.

Important note

This change has to be added back after every Cumulative Update, because Exchange setup can overwrite web.config. If you still maintain an older on-premises Exchange environment, keep this on your upgrade checklist.

Current note

Microsoft still documents this as an OU limit issue in ECP, and the same workaround is still described in their troubleshooting guidance. The historical part of this post is the product version: Exchange Server 2013 support ended on April 11, 2023, so if you are still seeing this there, it is also a reminder that the platform itself is now out of support.