In version 8.6.1 a new parameter was introduced in update 20110504EX, called "NoTargetExchangeConnection". From the hotfix Release Notes:
Support for a new mail source agent configuration parameter, NoTargetExchangeConnection, has been added in this update. This parameter should be specified in the mail source agent's config.ini file. The parameter can have one of the following values:
Mailbox switch is performed without connections to the target Exchange server. The LastPSTAppliedSwitch parameter is ignored in this case.
Important: The agent does not verify that the target mailbox exists, and does not put a switch notification in the target mailbox. Use this parameter with caution, because it may result in a switch to a non-existent mailbox and mail flow may be broken. A value of 1 should be used only if the connection from the source environment to the target Exchange server is restricted.
The mail source agent behaves as in versions prior to the update.
This parameter is not available in version 8.7.
In version 8.8 the required parameter has changed name to "SuppressMailboxValidatingOnSwitch=1"