An Email is not analogous to a house, it's more akin to a tent, or a lean-to; A simple solution to a simple problem.
Tent: How can we build a serviceable shelter that can be quickly un/packed, built, and costs next to nothing?
Email: How can we enable everyone to send messages to everyone else with as little inconvenience as possible?
Both solutions solve the stated problem, and that's it. If I want a secure shelter, a tent alone will not suffice, I have to build a ditch&wall around it. Similarly, if I want secure communication, I have to encrypt the message before sending it via Email.
SMTP is not made to be secure, because that's not the problem domain it concerns itself with.
An Email is not analogous to a house, it's more akin to a tent, or a lean-to; A simple solution to a simple problem.
Tent: How can we build a serviceable shelter that can be quickly un/packed, built, and costs next to nothing?
Email: How can we enable everyone to send messages to everyone else with as little inconvenience as possible?
Both solutions solve the stated problem, and that's it. If I want a secure shelter, a tent alone will not suffice, I have to build a ditch&wall around it. Similarly, if I want secure communication, I have to encrypt the message before sending it via Email.
SMTP is not made to be secure, because that's not the problem domain it concerns itself with.