When your disk quota is used up, this is usually caused by the default account email address filling up with spam. By default, if an email address does not exist, cPanel will send the email to the default account. For example, if your cPanel username is exampl and your domain is example.com, your default email will be exampl@example.com. If someone sends an email to johndoe@example.com but this email does not exist as either a mailbox or forwarder, cPanel will send this email to the default email account.

Disable default email account

1. Login to cPanel and click on email accounts.
2. Select manage next to the default account.
   
3. Click on Manage Default Address
   
4. Select Discard and press Change.