I recently joined Microsoft as a Premier Field Engineer for Messaging. One of the top issue's i started seeing are mailboxes that are converted to linked or shared mailboxes after migration from Exchange 2003, whereas they should be plain user mailboxes.

While it's no problem to convert a shared mailbox to a user mailbox - Set-Mailbox UserMbx1 -Type Regular - you will have a hard time converting a large number of linked mailboxes to user mailboxes, as the official procedure calls for disconnecting and reconnecting (!) the mailbox.

Let's start and see why this happens and what we can do about it -

Back in the days of 5.5, every mailbox had to have a so called "Associated External Account" - or short "AEA". This was necessary because Exchange 5.5 had it's own directory service, and NT accounts had to be matched somehow. In Exchange 2000 / 2003 / 2007, the AEA account is mostly used for ressource forest deployments, where a disabled mailbox account needs to be matched with an enabled user account from a user forest.

The problem here is that most customers have always upgraded their enviroment since the earliest days - and therefore still have (obviously no longer existing) accounts carrying the AEA property in their mailbox rights ACL's - this probably looks something like below: More...