They're advising you to use a personal email that is tied more directly to your name or identity, like using Yahoo Mail, Outlook, or Gmail, which they view as more personalized.

If you'd like, you could try creating an email address with your actual name (or a variation) on one of the recommended services, like: yourname@gmail.com yourname@yahoo.com This should resolve the issue

Most "privacy" webmail providers, such as ProtonMail, Tutanota, and mainstream services like Gmail, Yahoo Mail, Outlook, and AOL Mail, are labeled as proprietary because they use closed-source software, meaning the code isn't publicly available for review. These services operate on controlled servers, with features and infrastructures managed entirely by the company. The proprietary nature allows them to implement unique features and monetization strategies, but it also requires users to trust the company’s privacy and security practices without independent verification. This contrasts with open-source alternatives where users can inspect and verify the code themselves.