Way back when, e-mail was from server-to-server, and really client-to-server as well, without much thought to security.
Messages were transmitted in plain text (no encryption) and the only people reading your mail (literally) were the system administrators who ran the email server.
Of course, those system administrators knew what was up and understood the later ramifications these open networks could have.
In 1991, Phil Zimmermann developed encryption software that would allow users to digitally sign and/or encrypt their e-mail in hopes of improving security on this open system.
Pretty Good Privacy (PGP) (RFC4880) was the name of his creation and the end result was that users could protect their data in a somewhat standard way.
Unfortunately, use was limited and it didn’t address the problem of a third-party gaining access to whom you were communicating with.
There had been rumors, and common sense should have told us as much, but it wasn’t until 2013 when Edward Snowden let the world know exactly how vulnerable our communications were did people start to pay attention.
It was at that point that we realized just what the U.S. Government, specifically the National Security Agency (NSA), had been up to.
The NSA, it was revealed, had been "partnering" with many information technology companies to help obtain data created and stored by individuals within the United States and around the globe.
To supplement this data, as sometimes they actually ran into businesses that wouldn’t quietly go along with their plans, they began to tap into portions of the Internet so they could capture the data as it was moving between the clients and servers and between servers as well.
While it hasn’t been a large focus in the United States, it should be remembered that if the U.S. Government was doing this, it is almost certain that other countries are too and who knows what they are doing with your data.
It’s difficult to accurately count the number of web servers that implemented secure connections after the revelation from Snowden but some polls saw a significant increase in adoption of HTTPS to help protect users.
The introduction of tools, such as Let’s Encrpt which launched in 2016, certainly helped to bolster the number of websites that supported HTTPS but many of these solutions didn’t address server-to-server connections or e-mail connections.
Fast forward to today, a new set of standards have been put in place to help protect e-mail, both while in transit (between the client and server as well as between servers) and while the email is sitting on the server.
It’s not exactly easy to understand how many of these "new" technologies work together or exactly how they protect your data.
There are many resources on the Internet to explain them to you; lets add another.