In your Exchange 2016 environment, you might have noticed the following Event (Event ID 9646) being logged:

• Mapi session (redacted) with client type MoMT exceeded the maximum of 32 objects of type Session

To resolve this issue, you can create a registry key on the server and then restart the Exchange Information Store.

• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem\Maximum Allowed Sessions Per User

The value for “Maximum Allowed Sessions Per User” Value data set to 500 and ensure it is set to a Decimal Value:

Once done, you can go to Services and restart the Exchange Information Store or reboot the server for the changes to take effect.

Hope it helps.