Awesome Privacy

An open-source, end-to-end encrypted anonymous email service. ProtonMail has a modern easy-to-use and customizable UI, as well as fast, secure native mobile apps. ProtonMail has all the features that you'd expect from a modern email service and is based on simplicity without sacrificing security. It has a free plan or a premium option for using custom domains (starting at $5/month). ProtonMail requires no personally identifiable information for signup, they have a .onion server, for access via Tor, and they accept anonymous payment: BTC and cash (as well as the normal credit card and PayPal).

Free and open source email service based in Germany. It has a basic intuitive UI, secure native mobile apps, anonymous signup, and a .onion site. Tuta has a full-featured free plan or a premium subscription for businesses allowing for custom domains ($12/ month). Tuta does not use OpenPGP like most encrypted mail providers, instead they use a standardized, hybrid method consisting of a symmetrical and an asymmetrical algorithm (with 128 bit AES, and 2048 bit RSA). This causes compatibility issues when communicating with contacts using PGP. But it does allow them to encrypt much more of the header data (body, attachments, subject lines, and sender names etc) which PGP mail providers cannot do.

An open source, privacy-focused, encrypted email service supporting SMTP, IMAP, and API access

Mailfence supports OpenPGP so that you can manually exchange encryption keys independently from the Mailfence servers, putting you in full control. Mailfence has a simple UI, similar to that of Outlook, and it comes with bundled with calendar, address book, and files. All mail settings are highly customizable, yet still clear and easy to use. Sign up is not anonymous, since your name, and prior email address is required. There is a fully-featured free plan, or you can pay for premium, and use a custom domain ($2.50/ month, or $7.50/ month for 5 domains), where Bitcoin, LiteCoin or credit card is accepted.