As you probably know, a linked mailbox is a mailbox that is associated with an external account. If you deliberately create linked mailboxes because you need them, good! The problem is that, sometimes, mostly after an Exchange 2003 migration, you may find out that some of the migrated mailboxes are now Linked Mailboxes on Exchange Server 2007.
The supported method to convert them to regular mailboxes involves disabling those mailboxes and then reconnecting them to the original account (read this article). But if you feel brave enough and want to save some time, here's the unsupported method:
- Run ADSIEdit and find the account to be modified;
- Clear the msExchMasterAccountSID attribute;
- Change the msExchRecipientTypeDetails from "2" to "1"