E-mail itself is a completely insecure message service. Mails are routed through all participating servers on the world, everyone can send them. And whatever washes up in your inbox could come from anyone. It is not that bad any more, the connection from sender to sender's provider and recipient and recipient's provider is usually transport layer encrypted. The connection between the providers is becoming encrypted as well, but there is no guarantee. Essentially the providers can still read the messages, which is unacceptable for confidential messages.
In order to mitigate this problem and introduce some end-to-end encryption, e-mail protocol extensions like S/MIME and PGP have been invented. Both basically work with key pairs that everyone has to have and some vetting mechanism. In the S/MIME world the key pairs are called certificates and the vetting is done via central authorities (just like with HTTPS-certificates in the browser). PGP uses a “web of trust”, where people have to sign each other's keys to provide trust via trusted parties.
The user experience of both has been catastrophically bad. Some e-mail clients supported S/MIME, I believe that Outlook and Thunderbird did and do so. Obtaining certificates was a big hurdle. When I went to school, I didn't afford to buy one, and none of my peers did either. It was just something which was useless to have unless everyone else also has one. Instead I tried PGP in the form of GPG, but except from select other nerd friends, nobody cared for it. A plugin was needed in Thunderbird, and often enough it did not work. Keys had to be revoked, people encrypted with the wrong key (often their own), and so on. In theory, both are completely secure end-to-end solutions. In practice their are useless as people don't have certificates.