KMail contains extensive support for the cryptography standard OpenPGP (encryption and signatures); KDE even contains a dedicated application for managing OpenPGP keys and using OpenPGP outside emails: KGpg. OpenPGP is free (both as in free beer and as in free speech) and very flexible; many other mail clients support OpenPGP either directly (like KMail) or via add-ons / plugins (e.g. Thunderbird; with certain limitations even Outlook).
It is desirable that you make yourself familiar with this subject. Using it is less complicated than you probably guess. Especially if you get help with the setup then it is possible even without in-depth knowledge to use the core functions without trade-offs in security.
In Germany there is a non-commercial project for supporting and listing free OpenPGP courses. The projects aims at both beginners (offering help to them), experts (for getting more people help beginners) and people of any expertise for supporting the propagation of OpenPGP on an organizational level. This is the international site. As soon as similar projects exist for countries connected to this language they will be listed here.
Not as useful as courses but as something to start with: