- Login to your cPanel account.
- Go to Email section and click on Email Accounts.
- On the next page you will get list of all your email accounts and option to create new mail as well.
- Scroll down to Email accounts section where list of all accounts will appear with their disk usage. Refer to the image. https://prnt.sc/uC99RMrJNM8Q
You can easily find the email disk usage of your custom email accounts inside cPanel.