I use GnuPG, a free implementation of PGP, to sign and encrypt emails - and I would prefer if more people would do the same (for instance by using enigmail for Thunderbird or gpg4win for Outlook), to enable the verification of my emails and to prevent people to read things which are not for their eyes.

My public key (ID 10AEE42F) is available here, via the awesome keybase.io (cross-linked to Twitter, GitHub and this domain) and on the usual keyservers; its fingerprint is 94D2 7AF6 7971 5659 DBED BB9C 58BA 7CA3 10AE E42F.

Last but not least, the Free Software Foundation published an excellent step-by-step guide to get you started with email encryption, called 'email self-defense'