Attempting to manage the email disk usage for a cPanel account results in this error:
Failed to lookup mailbox status: Mailbox doesn't exist: INBOX.$EMAIL_USER@DOMAIN_TLD
The non-existent email address(es) specified in the error has a symlink still existing within
/home/$USER/mail that is responsible for the error.
Delete the symlink(s) to the non-existent email address(es) within