Mapi session "/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=username" exceeded the maximum of 16 objects of type "session"

Event ID 9646 is logged in the application event log of your Exchange Server 2010 computer when a client opens many MAPI sessions

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
3.If the Maximum Allowed Services Sessions Per User entry does not exist, do the following:
     a. On the Edit menu, point to New, and then click DWORD Value.
     b. Type Maximum Allowed Services Sessions Per User as the entry name, and then press ENTER.
4. Right-click the Maximum Allowed Services Sessions Per User entry, and then click Modify.
5. Click Decimal, type the value that you want to set in the Value data box, and then click OK.
6. Exit Registry Editor.
7. Click Start, click Run, type services.msc in the Open box, and then click OK.
8. Click the MSExchange Information Store service, and then click Restart Service.

You can find a solution to earlier versions of Exchange but it doen's work on Exchange 2010. The difference is Maximum Allowed SERVICES Sessions Per User instead of Maximum Allowed Sessions Per User.