Outlook 2013 works differently, but Legacy clients need modifications to server side.

In Exchange 2013:

Create Throttling Policy and apply it to all mailboxes:

Then add registry key (below is for 5000 concurrent connections):
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem]”Maximum Allowed Sessions Per User”=dword:00001388
Then restart Information Store service.