I’ve used PGP (Pretty Good Privacy) since I was in college. It provides for both digital signatures and strong encryption and content without the user having to go make extraordinary effort. The process uses what is known as Public Key Encryption and uses a Web Of Trust to certify individual users.
For years I used the original PGP 2.6.2, 5.x and 6.x products that were available as freeware. After PGP was acquired by a much larger commercial entity, most development has shifted to the open-source community that makes it available as GnuPG aka GPG.
There are several plugins available for common Email Clients such as Thunderbird and Outlook to natively integrate the functions into those applications. Additionally plugins are available for Firefox to enable encryption and signing of WebMail services such as GMail (Google Mail).
I look forward to your signed/encrypted emails,