Many domain provider such as namecheap, godaddy, google offers free email forwarding services for their customer. But this is only applicable if you use their DNS. If you migrated DNS out to say CloudFlare then you have to setup email forwarding on your own.
Email forwarding is a method which redirects emails sent to your domain, and sent it to your personal email account of your choice. Therefore, you can receive emails from your custom domain without exposing your personal email address.
It offers a few advantage over a traditional mailbox: