Increase the maximum MAPI session limit from 32 concurrent to 5000 concurrent
Outlook 2013 works differently, but Legacy clients need modifications to server side. In Exchange 2013: Create Throttling Policy and apply it to all mailboxes: 
New-ThrottlingPolicy multiplemailboxes Set-ThrottlingPolicy multiplemailboxes -RCAMaxConcurrency Unlimited -EWSMaxConcurrency Unlimited -EWSMaxSubscriptions Unlimited -CPAMaxConcurrency Unlimited -EwsCutoffBalance Unlimited -EwsMaxBurst Unlimited -EwsRechargeRate Unlimited get-mailbox | Set-Mailbox -ThrottlingPolicy multiplemailboxes
Then add registry key (below is for 5000 concurrent connections):
"Maximum Allowed Sessions Per User"=dword:00001388
Then restart Information Store service.