Short explanation: network effects and backwards compatibility. When the SMTP protocol was designed/evolved, spam and other malicious uses did not exist. They

Short explanation: network effects and backwards compatibility. When the SMTP protocol was designed/evolved, spam and other malicious uses did not exist. They didn't even exist for many, many years afterwards. When the spam really became a problem, everyone was already using smtp and switching the whole world to an alternative wasn't (and still isn't) possible. Designing email protocol that would eliminate spam is not hard if you start with such goal in mind. The people who designed smtp were very smart (it is being used decades later), but they were pioneers and unsurprisingly didn't anticipate all of the consequences. When they were designing it, there number of people using the system were in thousands. I doubt anyone in early eighties was anticipating the magnitude of smtp's success and the abuses of the system only became profitable when email started being used by millions.