The mailbox for the default email account seems to have no quota. It fills up to the point that it shuts down the FTP service(can’t upload files) and the disk usage for the account goes berserk.
The issue can be resolved by setting your main account to discard email.
This can be done via :
cpanel>>mail>>set default address
It is advised to go to ‘Advanced Settings’ and set it to discard without reply. This will prevent you from sending out large amount of replies which can look like spam to providers.