If you are getting this error, Please check the following from your cPanel.

• On your right, there is a resource usage summary. Confirm that your disk space is not full.
• If the disk space is alright, scroll down to the email section and go to email accounts.
• Check the quota of the email account affected. You can also check for any other email that has a high usage warning.
• Change the disk quota to unlimited or add to your preferred limit.

If the error doesn’t clear. Kindly contact the support team for further assistance.