There are various software packages that offer security features and encryption for client based email (i.e. non web-based email). Try these links for some of the options:
AB Text Encrypter http://ab_software.tripod.com/app/abte.htm
CryptoHeaven, From $66 per year www.cryptoheaven.com
PMBx $55 per year https://www.pmbx.net
Protonmail FREE https://protonmail.com
SecureNym $60 per year www.securenym.net
Swissmail €30 per year www.swissmail.org
Email SecurityA helpful guide from Cloudwards to keeping your inbox safe.