Event ID - 9166

Event Id9166
SourceMSExchangeAdmin
DescriptionFailed to log on to the MAPI session on server ServerName Error: Access is denied.
Event InformationAs per Microsoft :
CAUSE
This problem may occur if both the following conditions are true:
You perform the move mailbox operation in a mixed-mode administrative group.
The account that you use to perform the mailbox move operation, or the Exchange 2003 Full Administrator account, is different from the Microsoft Exchange Server 5.5 service account.
This problem is caused by a timing problem. The timing problem may occur if your Exchange Server 2003 computer is busy when you try to move the mailbox. In this scenario, the move mailbox operation may be unsuccessful because the Exchange Full Administrator account may not have sufficient rights on the Global\ExchangeAdminMapiLogon mutual exclusion object (mutex) to perform a MAPI logon.
Typically, many Microsoft Exchange System Attendant service threads use the Global\ExchangeAdminMapiLogon mutex. These threads all run under the Local System account. Therefore, these threads all have sufficient rights to acquire this mutex regardless of the thread that created the mutex. In this scenario, and in a pure Exchange Server 2003 administrative group, the Global\ExchangeAdminMapiLogon mutex is created in the following manner. The following accounts have permissions:
Local System
Local Administrator or the Administrators Group
However, in a mixed administrative group, this mutex could be created by one of the mad.exe threads that impersonates the legacy Exchange Server 5.5 service account for free/busy interoperability. When this behavior occurs, the mutex is created in the following manner. The following accounts have permissions:
Local System
Exchange Server 5.5 service account
Therefore, in this scenario, your Exchange Full Administrator account may not have sufficient permissions to perform the move mailbox operation.
WORKAROUND
To work around this problem, use one of the following methods.
Move the mailbox from a computer that is not running Exc
Reference LinksA move mailbox operation is not successful, and Event ID 9166 appears in the Application log in Exchange Server 2003

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