iCloud email not working despite deleting emails and sufficient storage

My daughter could no longer send or receive iCloud emails. We deleted everything from the inbox, junk, trash etc and she is part of the iCloud+ family share plan that gives her more than enough storage. After all this her email still doesn’t work. Even tried turning the phone off and on. Any ideas?

[Re-Titled by Moderator]

Original Title: iCloud email over quota