If you receive this error message when trying to access Webmail (www.your-domain.com/webmail), it means the email address has no allocated space left or the entire hosting account has no disk space available.
To see the exact cause, log in to cPanel at www.your-domain.com/cpanel and on the right side you will see the used space out of the total allocated space (Disk Usage). See the image below for example.
If you have available space in the entire hosting account, it means the specific email address has no space left.