Awesome Privacy

Free and open source email application developed and backed by Mozilla -it's secure, private easy and customizable. As of V 78.2.1 encryption is built in, and the TorBirdy extension routes all traffic through the Tor network. Forks, such as Betterbird may add additional features.

Productivity-based email client, for Windows and MacOS. eM Client has a clean user interface, snappy performance and good compatibility. There is a paid version, with some handy features, including snoozing incoming emails, watching for replies for a specific thread, message translation, send later, and built-in Calendar, Tasks, Contacts and Notes. Note, eM Client is proprietary, and not open source.

Simple, modern, fast web-based mail client. This is an IMAP-only fork of RainLoop that mitigates a severe RainLoop vulnerability and adds several new features.

Browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an email client, including MIME support, address book, folder manipulation, message searching and spell checking.

K-9 (or Thunderbird for Android) is open source, very well supported and trusted - k9 has been around for nearly as long as Android itself! It supports multiple accounts, search, IMAP push email, multi-folder sync, flagging, filing, signatures, BCC-self, PGP/MIME & more. Install OpenKeychain along side it, in order to encrypt/ decrypt emails using OpenPGP.