Custom domains offer you a way for sending messages from other addresses other than @protonmail.com. You can use your own domain with ProtonMail, and send messages from the addresses you add.
If you have upgraded to a premium plan, you can use a custom domain within ProtonMail and enjoy all the benefits of seamless end-to-end encryption! You must have access to the domain’s DNS records so that you can change them to point to ProtonMail. Unfortunately, this means you will not be able to add addresses from domains you do not own or control, such as .edu or .gov domains.
If you don’t have your own domain yet, you can buy one through domain registrars like namecheap.com. Domains that are not taken yet can be quickly acquired and typically cost around $10 per year. Let’s assume you have the domain funoccupied.com, then you can set your ProtonMail account to send and receive email with addresses such as email@example.com. Learn more about setting up a custom domain here.