Awesome Privacy

An 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 and desktop email clients, anonymous signup, and an encrypted calendar. Tuta has a full-featured free plan and premium subscription plans allowing for custom domains (starting at $3/month). Tuta does not use OpenPGP like other encrypted mail providers, instead they use a standardized, hybrid method consisting of symmetrical and asymmetrical algorithms (with AES256, and RSA 2048 or ECC (x25519) and Kyber-1024). 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. The recent upgrades to Tuta's encryption algorithm makes data stored and sent with their service safe against attacks posed by quantum computers.

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.

A Berlin-based, eco-friendly secure mail provider. There is no free plan, the standard service costs €12/year. You can use your own domain, with the option of a catch-all alias. They provide good account security and email encryption, with OpenPGP, as well as encrypted storage. There is no dedicated app, but it works well with any standard mail client with SSL. There's also currently no anonymous payment option.