KMail
KMail is an email client for different GNU/Linux distributions, which was developed by KDE and is part of an "information management suite" named Kontact. It is offering OpenPGP-encryption and implements the WKD-standard.
How to enable WKD
Here is a report to enable WKD by default, so that more users notice that they could easily encrypt to some of their recipients.
- https://bugs.kde.org/show_bug.cgi?id=472426 (Enable checking if OpenPGP encryption is possible by default)