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
in version 5.18.3
- In the menu open "Settings" and click on "Configure KMail..."
- In the next window that will be opened select the account, for which you want to enable WKD, and click on "Modify..." on the right
- Open the tab "Cryptography" and activate the checkbox "Automatically encrypt messages when possible"