Outlook client không connect được Exchange server bằng Exchange mode
Log trên Server:

Event Type: Error
Event Source: MSExchangeIS
Event Category: General
Event ID: 9646
Date: 6/19/2013
Time: 10:44:48 AM
User: N/A
Computer: XXX
Description:
Mapi session “/o=First Organization/ou=First Administrative Group/cn=Recipients/cn=xxx” exceeded the maximum of 32 objects of type “session”.

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Nguyên nhân:

Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer when a client opens many MAPI sessions

Cách Fix:
Trên Exchange Server

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

Mặc định số sessions cho phép là 32
Set sessions thành 200 (tùy chỉnh vào bạn)