We recently migrated from Exchange 2010 to Exchange 2013. Our receptionist manages the calendars for our executives using Outlook 2010. She can send/receive email and modify her calendar without issue. Occasionally she receives an error message when attempting to update another user's calendar. "...Your server administrator has limited the number of connections you can open simultaneously...." Once Outlook 2010 is restarted the issue goes away for a while. Updating calendar's from OWA is performed without issue.
In troubleshooting I have added the registry key for "Maximum Allowed Sessions Per User = 320". With limited positive result.