"Your server administrator has limited the number of items you can open simultaneously" error and you can't expand public folders in Outlook

2025-06-25
Applies to: Exchange Online, Exchange Server 2019, Exchange Server 2016, Exchange Server 2013, Microsoft Outlook 2016, Microsoft Outlook 2013

Symptoms

Users who have sufficient permissions to public folders receive the following error message when they expand a public folder tree in Microsoft Outlook or Outlook on the Web (OWA):

Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.

Cause

The following conditions can cause this problem to occur:

You're using an outdated version of Microsoft Outlook 2016 or 2013.
The default public folder mailbox resources are exhausted from servicing the public folder hierarchy. However, the threshold at which Exchange can provision a new public folder mailbox (through Auto-split) hasn't been reached.

Resolution

If the problem occurs in only Microsoft Outlook 2016, install the April 5, 2016, update for Outlook 2016.

If the problem occurs in only Microsoft Outlook 2013, install the April 5, 2016, update for Outlook 2013.

If several users report the problem in both Outlook and OWA, follow the instructions for the relevant scenario.

Scenario 1

You're using the DefaultPublicFolderMailbox property to predefine a hierarchy for servicing a public folder mailbox. For example, you may have predefined the default public folder mailbox for user mailboxes in a remote site so that users can connect to the specified public folder mailbox based on their geographical location. The problem occurs if there are user traffic spikes on the default public folder mailbox.

To resolve this problem, create a new public folder mailbox, and manually update the public folder mailbox hierarchy to be ready to use that public folder mailbox to service the public folder hierarchy for some of the affected users. Here's how to change the DefaultPublicFolderMailbox property for these user mailboxes.

Connect to Exchange Online PowerShell.

Run the following command to create a public folder mailbox:

New-Mailbox -Name "<New public folder mailbox name>" -PublicFolder

Run the following command to update the public folder hierarchy for the public folder mailbox

Update-PublicFolderMailbox -InvokeSynchronizer -Identity "<New public folder mailbox name>"

Run the following command to find the user mailboxes whose DefaultPublicFolderMailbox property is set to the predefined public folder mailbox:
```
Get-Mailbox | where DefaultPublicFolderMailbox -like "*<Public folder mailbox name>*" 
```
In the result, note the value of the <User mailbox name> parameter.
Run the following command to set the DefaultPublicFolderMailbox property to the new public folder mailbox for the user mailbox that you noted:
```
Set-Mailbox -identity "<User mailbox name>" -DefaultPublicFolderMailbox "<New public folder mailbox name>"
```

Scenario 2

You're using the DefaultPublicFolderMailbox property to predefine a hierarchy for servicing a public folder mailbox. There are other public folder mailboxes that are ready to service the hierarchy for users and that have no special configuration. This is the case if all users are in the same geographical location.

In this scenario, find the user mailboxes whose DefaultPublicFolderMailbox property is set to non-null values, and set the property to null. This will redirect users to a relatively low-load public folder mailbox.

Here's how to change the property:

Connect to Exchange Online PowerShell.
Run the following command to find the user mailboxes whose DefaultPublicFolderMailbox property is set to non-null values:
```
$users = Get-Mailbox | where DefaultPublicFolderMailbox -ne $null 
$users | ft name, *effective**public*, *default**public* 
```
In the result, note the value of the <User mailbox name> parameter.
Run the following command to set the DefaultPublicFolderMailbox property for these user mailboxes to null:
```
$users | Set-Mailbox -DefaultPublicFolderMailbox $null 
```

Run the following command to verify that the value is set successfully:

Get-Mailbox <User mailbox name> | fl name, *effective**public*, *default**public*