This is how to fix invalid email disk space usage (maildirsize)
- Login to your server via FTP
- Navigate to the following path /mail/yourdomain.com/user
- From this directory, delete the file named maildirsize
- Once this file is deleted, the system should reform the mail account automatically with the correct mailbox quota size upon new email activity.
Notes: If the correct mailbox quota is still not being displayed, from within cPanel, you can navigate to email accounts and click the change quota button to set a new quota.
Source: http://www.lancelhoff.com/error-invalid-maildirsize-file/