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How can I increase the session limit for simultaneous MAPI access?


The Microsoft Exchange Server allows a maximum of 32 concurrent sessions to access MAPI.  

When private and public Exchange Server adapters are connected, more than 32 sessions may be opened on the MAPI interface. 

If this is the case, the following message with source "MSExchangeIS " can be found in the Windows Event Viewer under Applications 
Close the MAPI session "/ o = Organization / Ou = Administrative Group / Cn = Recipients/Cn = Recipient "because the maximum of 32 objects of the type "Session" has been exceeded. 

The error "MAPI_E_FAILONEPROVIDER" is displayed in the Virtual Directory configuration interface

If the error occurs on the Virtual Directory configuration interface and no corresponding error is listed in the Windows Event Viewer, it may simply be an authorization problem of the user under which the Virtual Directory service is running to the MAPI interface. 

To increase the maximum number of sessions, the following value must be adjusted in the Windows registry on the Exchange Server. In this example, the maximum number of concurrent sessions is increased to 128 sessions: 

"Maximum Allowed Sessions Per User"=dword:00000080 

Afterwards the service "Microsoft Exchange Information Store" must be restarted.  
During the restart, the mailboxes are not accessible. 

There is also an NSPI session limit, which Microsoft has introduced from Windows Server 2008 onwards as an operating system (also for 2003 domains) based domain controllers. This allows a maximum of 50 sessions per user and domain controller by default. 

To increase this limit, the following value must be set in the Windows registry on each domain controller: 

"NSPI max sessions per user"=dword:00000200 

It is recommended to perform a restart of the domain controller. 

Further details can also be found in the Microsoft Knowledgebase:

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