I know many people probably rely on web-based email clients, but for those of us that still like having a separate application to manage email, Thunderbird remains an excellent choice. It’s from the same suite of software as Firefox which also ensures their dedication to privacy and security.
The latest update to version 78 has added in complete support for End-to-End Encryption (E2E Encryption) relying on OpenPGP. They have a brief article on E2E encryption here: https://support.mozilla.org/en-US/kb/introduction-to-e2e-encryption
Thunderbird can be downloaded here: https://www.thunderbird.net/en-US/ and is available for Windows, Mac, and Linux.
I’ve been using it… for at least 14-15 years. It never got as much attention as Firefox and there was one point where it seemed as if Mozilla was going to drop it. But it seems to have gotten a new lease on life and there have been a bunch of solid updates over the last couple of months.