The suite of protocols supporting email infrastructure, like SMTP, POP, were an open standard and operated on plain text, for anyone to adopt since the beginning. The inventors at ARPANET originally intended it for closed communities where all addresses could be trusted, and they wanted everyone to be able to bootstrap and maintain their email.

Email till date is the most obvious protocol to connect any two incompatible systems as they only assume of each other to be capable of reading plain text over TCP, and they all do.

But the inventors had not foreseen that their project would break out of small trusted communities into the zero-trust masses, become an essential utility, and since then everyone using it would forever have to defend themselves from spam. So, they never thought of the need for encrypting the email headers for privacy.

Spam scale and incentives made email practically no longer an open utility. It's not free either if people factor privacy into their equation.