Why can I not delete a mailbox as an admin user?
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By default, the permissions on newly created mailboxes do not allow
access to any other user. Therefore even root user can not delete
mailbox.

To delete a mailbox, you need to give appropriate rights with the
command "sam user.mailbox username all" after that you can delete the
mailbox. Even just giving the user the "c" right (Create and Delete
mailbox) is enough.
