I read "If you're experiencing a 'mailbox unavailable' error when logging in, we can clear this by updating the mailbox on our systems. Please create a new post and a member of staff will be in touch within 24 hours to help further. Please don't post your email address in the initial post, we'll ask you for this via private message when we reach out."
This problem has been going on for 3 WEEKS. I have phoned 4 times. The problem has cleared for a few days then returned.