If Mail won't launch, and a message like "Mail cannot save information about your mailboxes because there isn’t enough space in your home folder" is displayed.
Check home directory access/permissions issue, then try deleting the file: Envelope Index-journal In folder: ~/Library/Mail Or: ~/Library/Mail/V6/MailData Or: substitute the correct V number for newer versions
Or from the terminal: cd ~/Library/Mail Or: cd ~/Library/Mail/V6/MailData Or: substitute the correct V number for newer versions (as above) Then: rm Envelope\ Index-journal
That will force the sqlite database to reindex and a corrupt index can cause this error.
TIPS PAGE