ProtonMail's custom domains feature allows users to use their own domain name for their email address instead of the default "@protonmail.com". This means that instead of having an email address like "yourname@protonmail.com", you could have "yourname@yourdomain.com". This feature is particularly useful for businesses and professionals who want to maintain a consistent brand image across their communications. It allows them to combine the security and privacy benefits of ProtonMail with the professional appearance of a custom domain. To use a custom domain with ProtonMail, you need to have a paid ProtonMail account and own a domain name. You then configure the domain's DNS settings to point to ProtonMail's servers. Once this is done, you can send and receive emails using your custom domain through ProtonMail's platform.