Exchange 2016- EVENT ID 9646 – MoMT exceeded the maximum of 32 objects of type Session

Here is another issue I encountered during my Exchange 2016 migration. A hand full of users have been getting knocked out of public folders with an Event ID 9646 and they have exceed the max of 32 session objects. What the hell does that mean? Why only a few users?

Investigating Event 9646

The Monday after I migrated public folders to Microsoft’s new “Modern Public Folders” a hand full of users began calling me they were unable to access them. Their specific error was:

“Cannot expand the folder. 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.”

Nothing I did on the workstation side of the equation helped the user out. I tried to prevent syncing of folders from other mailboxes, deleted the Outlook profile and recreating it and even a simple reboot. Nothing worked!

On the server side I did discover that recycling IIS (IISRESET) alleviated the issue temporarily, until the next user called with the same issue.

What was going on here?

According to Microsoft each user by default was allowed 32 objects (or connections) to the server to help control the amount of resources a user could consume during an outlook session. Once that threshold was reached the user was cutoff and an error was displayed.

In the event log we would see Event ID 9646 with the following information

In order to fix this a registry key needs to be created that will allow more sessions per user.

Add the following:

Navigate to this registry location

Create a key (DWORD 32)
Maximum Allowed Sessions Per User

I set mine to 500 and it seemed to have done the trick!
You must bounce the Microsoft Exchange Information Store service for this to take effect, or reboot the server.

You can import this into your registry:

Windows Registry Editor Version 5.00

"Maximum Allowed Sessions Per User"=dword:000001f4

I hope this helps with your 9646 errors!

Thank you for reading my blog,


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