A few months back, I noticed that a user was unable to access a shared mailbox because Outlook needed a password for it. The computer had been setup with entirely separate Exchange accounts instead of adding permissions for the user to add the mailbox to their own account. I decided to change this and had a lot more trouble than I would have expected.
Before removing the extra account in Outlook, I went to add the required permissions. Exchange 2013 removed the GUI interface for managing Full and Send As permissions, so I ran the following command:
When I went back to Outlook, The accounts had been duplicated and despite removing the accounts that were previously setup, I would get errors when trying to open one of the two duplicates. Even creating a new Outlook profile didn’t make this go away. After some more research, I found that there is a parameter that can be added to the Add-MailboxPermission command to turn Auto Mapping off and Exchange will automatically add the account to your Outlook Account. While Auto Mapping is great, it was causing all kinds of problems with this account. I had to the Remove-MailboxPermission and then re-add the permissions with Auto Mapping set to false. After that everything worked again.