In an enterprise, there are many people who use the same Exchange mailbox, who configure Outlook clients on their machines to connect to Exchange 2010 using the MAPI protocol.
The default Outlook MAPI way to connect to an Exchange server, the maximum number of sessions is 16, more than 16 users at the same time using an Exchange mailbox account to connect to exchange, there will be a part of the user can not connect the situation.
For details, refer to the official article:
Exchange Store limits:exchange Help
https://technet.microsoft.com/en-us/library/ff477612 (v=exchg.141). aspx
In MicrosoftExchange Server 2010, Connection and usage restrictions are imposed on the Exchange store to prevent a single application or individual user from using all available connections to the Exchange store. If you allow a single user or application to use all connections, other users or applications will not be able to access the Exchange store, which can lead to downtime.
Concepts of sessions and threads
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Session
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A session represents a connection that is used by service and client applications, such as Microsoft Outlook, to connect to an Exchange store. Services and clients can have multiple sessions at specific times. The terms "connection" and "session" can be used interchangeably.
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Thread
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A thread represents a request to execute concurrently with the Exchange store. For example, if a user opens a folder in Outlook, Outlook executes the request on behalf of the user for Exchange. The request executed is a single thread.
For example, 75 users who log on to a server at the same time are equivalent to 75 sessions. However, in these 75 sessions, only 5 sessions can be requested through a thread.
In the above article, the way to modify the maximum number of sessions is given.
At the same time, we can also use some administrative means to deal with the problem, such as many people use a common computer to log in the mailbox.
This article from "Zeng Hung Xin Technical column" blog, declined to reprint!
Tip:outlook the session limit for a MAPI client to connect to an Exchange 2010 server