If you’ve found this article then you probably already have your own reasons for wanting to disable auto-mapping, but I will explain why it has been useful for some of our clients. When you grant full access permissions to a user for a certain mailbox, the mailbox will be automatically added to the users Outlook session – assuming it is Outlook 2007 or newer.
Sometimes it is preferable to add the account separately and not have it auto-map. For example, if a user is working from an office with a particularly slow Internet connection, or a user temporarily borrows a lower spec machine for the day. Additional mailboxes put extra load on both the Internet connection and the computer. In this situation enabling cached mode is an option but not always available.
This cannot be done in the EMC (Exchange Management Console) so here is the Powershell command. The percentage symbol is showing the variables which need to be removed before running the command...
Add-MailboxPermission -Identity %email@example.com% -User %firstname.lastname@example.org% -AccessRight FullAccess -InheritanceType All -Automapping $false
It is important to note that you will need to ensure your Exchange 2010 server is running at least SP2.