The user-friendly, lightweight, and fast email client Latest Release: 3.18.0 / 4.0.0 (News), on 10 July 2021

The GPG plugin

The GPG plugins (PGP/Core, PGP/Mime and PGP/Inline) provide digital signature and encryption features: Every account can be configured with different defaults regarding signature and encryption, making it easy to quickly switch between sets of parameters.
Although deprecated, the PGP/Inline method of signing or encrypting emails is still available, so you can switch to it for correspondents who still have older mail user agents that cannot handle the OpenPGP/MIME standard.

GnuPG and GPGME are required, http://www.gnupg.org/

Here are a few screenshots illustrating the features: