ProtonMail

OpenPGP

Tor-friendly

ProtonMail is an end-to-end encrypted email service founded in 2013, by scientists who spent time at the CERN research facility. ProtonMail uses client-side encryption to protect email content and user data before they are sent to ProtonMail servers.

Notes:
ProtonMail, based in Switzerland, is accessible via Tor, accepts Bitcoin (upon request), and supports 2-step authentication.

You should also note that when ProtonMail users email each other, the emails are automatically encrypted. ProtonMail also has integrated OpenPGP encryption to encrypt emails sent to non-ProtonMail users.

StartMail

OpenPGP

StartMail is a private email service developed by the creators of Startpage. StartMail safeguards your data and email activity with state-of-the-art security and technology. They also promise to never read your email.

Notes:
StartMail allows you to create unlimited email aliases that you can create & delete at any given time. This helps to hide your actual email address from services you use.

Email Encryption is also integrated into the StartMail Webmail. You can also set a password for recipients that don't use encryption. StartMail works on any device with a web browser and internet. More features are displayed here.

Tutanota

NO OpenPGP

Tutanota claims to be the world's most secure email service. Tutanota is easy to use, private by design, and encrypted. Tutanota, like ProtonMail, offers a free plan for you to get started, though if you are inactive for 6 months it will be removed.

Notes:
Tutanota is based in Germany. They allow you to use a custom domain when you purchase a paid plan with them. They support 2-step authentication, but they do not utilize OpenGPG on their standard webmail.

Tutanota does not operate a Tor Hidden Service, despite users requesting them to do so on multiple occasions. Tutanota says they will consider it, but we've seen no official updates on this since 2018.