If you get this message: "Mail cannot save information about your mailboxes because there isn’t enough space in your home folder"...
Check for a home directory access/permissions issue, then try deleting the file "Envelope Index-journal" in ~/Library/Mail or ~/Library/Mail/V6/MailData
Or from the terminal: either for older versions: cd ~/Library/Mail or for newer versions: cd ~/Library/Mail/V6/MailData Then: rm Envelope\ Index-journal
That will force the sqllite database to reindex which will help if a corrupt index has caused this error.