In 1971 the first ARPANET email was sent, and through RFC 561, RFC 680, RFC 724, and finally 1977's RFC 733, became a standardized working system.
Other, separate networks were also being created including:
Unix mail was networked by 1978's uucp, which was also used for USENET newsgroup postings
IBM mainframe email was linked by BITNET in 1981
IBM PCs running DOS in 1984 could link with FidoNet for email and shared bulletin board posting