When you send an email to someone, it goes through something called Simple Mail Transfer Protocol (SMTP), a standard that was first developed in the 1980s and that lacks the ability to fully encrypt our messages. Because of this, a group of engineers from several different companies — including from Microsoft, Google, Comcast and LinkedIn — are working on a new proposal that would update the standard to ensure full encryption for all email messages.
http://bgr.com/2016/03/22/google-microsoft-ietf-email-encryption/