Some mailboxes show up as Shared or Linked... after Migrating to Exchange Server 2007

Here's a scenario where after migrating users from Exchange Server 2003 to Exchange Server 2007, some of the user mailboxes show as "Shared" Mailbox or "Linked" Mailbox within the Exchange Management Console... instead of showing them as "User" Mailbox.

A Linked mailbox is a mailbox that is associated with an external account. Thus Linked mailboxes are normal mailboxes that can hold an External account in a separate resource forest.

Few of these mailboxes here were shown as Linked mailboxes after migration because an orphan Active Directory Account has the "Associated External Account" right on the mailbox in question.

To fix this and have them show up as normal "User" mailboxes remove that orphan Active Directory account that has the "Associated External Account" right on the mailbox in question and run the following command from the Exchange Management Shell:

set-mailbox -id MAILBOX_NAME -ApplyMandatoryProperties

Now we come to why few mailboxes are shown up here as Shared mailboxes. If you have Self permission with associated external account checked, you get the shared mailbox on migration. That is the reason these mailboxes are showed up as shared.

To fix this, remove the "Associated External Account" permission from the Self and again run that command:

set-mailbox -id MAILBOX_NAME -ApplyMandatoryProperties

To read and understand more about the ApplyMandatoryProperties Parameter, read this excellent blog entry from Evan Dodds.