After adding additional mailboxes to Outlook (Tools > Email Account > Change > More Settings > Advanced > Add), when you retrace your steps you may find that you are unable to remove the additional mailbox(es) because it isn’t listed, even though the mailbox is still visible in the main Outlook pane. When right-clicking on the mailbox and selecting “Close Mailbox – Name” you may receive the following error:
Navigate to the following in the registry:
HKCUSoftwareMicrosoftWindows NTCurrentVersionWindows Messaging SubsystemProfilesDefault
Look through each of the folders, and delete the folder that correspond to the additional mailbox(es) causing an issue.