https://en.m.wikipedia.org/wiki/Shiva_Ayyadurai ... I know, we have to assume no one actually invented it...it just poof, appeared.
It’s a good read.
RFC 196
https://tools.ietf.org/html/rfc196
A MAIL BOX PROTOCOL July 20, 1971
Here is a 1971 unix manual with the mail command
https://www.bell-labs.com/usr/dmr/www/man12.pdf