You use a MAPI or Collaborative Data Objects (CDO)-based application to log on to a mailbox in a Microsoft Exchange Server 2010 environment.
Your user account is the last logon object on the remote procedure call (RPC) connection.
The application does not send a remote operation (ROP) request for more than 2 minutes.
The application tries to log you off from the mailbox.
In this scenario, the Microsoft Exchange RPC Client Access service throws a ServerUnavailableException exception. Additionally, the RPC_S_SERVER_UNAVAILABLE (0x6BA) error code is returned in the application.
To resolve this issue, install the following update rollup:
2803727 Description of Update Rollup 1 for Exchange Server 2010 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.