A new client side feature which Microsoft has released in Exchange 2010 SP1 is the ability of Outlook 2010 to automatically map all mailboxes to which the logged in user has full access.
There by, on each mailbox that you have been granted Full Access, you will see it on your own personal mailbox.

In Order to disable automatic mapping:

  1. Go to this user mailbox, right click on it and choose manage full mailbox access, then just remove your user from the full mailbox access page
  2. You can use ADSIEDIT, and go this user properties, and under msExchDelegateListLink attribute, just remove the link to your user
  3. You can use the following script for future mailbox permissions – this will add full mailbox access with no AutoMapping.
  4. You can remove msExchDelegateListLink values from multiple users properties using ADModify.net

In order to find all users that have Full Mailbox Access on other users’ mailbox, run the following powershell:

Get-Mailbox -ResultSize Unlimited | Get-MailboxPermission | ? {($_.AccessRights -match "FullAccess") -and -not ($_.User -like "NT AUTHORITYSELF")} | ft Identity, User