How to fix the error "invalid maildirsize":
- Login to your server via FTP or File Manager.
- Go outside of public_html and navigate to /mail/yourdomain.com/email-user
- From this directory, delete the file named maildirsize.
- Once this file is deleted, and next time you use that email address, the system should reset the mail account with the correct mailbox quota.
If the correct mailbox quota is still not being displayed, you can navigate to cPanel > Email Accounts and click the Change Quota button to set a new quota.