This is a subject that I like to research from time to time. Occassionally, as I run across a resource I may update this list. If you wish to contribute information, please include source and email nate at this domain.
Electronic mail was first "invented" in 1965 by Tom Van Vleck, and implemented over ARPANET (the predecessor of the internet) in 1971 by Ray Tomlinson, who incidentally was the one who brought us the @ sign in email at that time.
RFC 561 in 1973 defined the email headers so different systems could pass messages more efficiently and by sometime that year fully 73% of all traffic on the ARPANET was electronic mail messages. By 1975 Steve Walker at ARPA actually created a message group (MsgGrp) whose sole purpose was to discuss standards in email headers because there were by that time, so many different email clients that it was getting difficult to manage including many commercial email packages by 1976. RFC 733 in 1977 was the result of this collaboration between many people.
1948 - JCR Licklider tenured professor at MIT
1956 - he left for BBN as VP and convinced them to buy a Royal-McBee digital computer for $30k
1960 - Ken Olsen of DEC contracted for BBN to test the new PDP-1 and compare to Royal-McBee
1960 - Paul Baran of RAND defined how to use message blocks to protect comm network under nuclear attack
1961 - Leonard Kleinrock at MIT - report on analyzing data flow networks, measuring data loss etc
1962 - DOD's ARPA hired Licklider for 2 years as IPTO manager who spread time-sharing theories into practice to tie government purchased mainframes around the county together, brings McCarthy and Minksi to BBN from MIT and Ed Fredkin pushes time sharing on PDP-1
1962 - AUTODIN start of development of military messaging system by Philo-Ford
1962 - BBN demos first time sharing PDP-1
1965 - Donald Davies at National Phsical Laboratory UK proposes packet switching which would be adopted into ARPANET
1965 - First computer to computer email on a single system- Tom Van Vleck with Noel Morris at MIT on the CTSS system
1965, Spring - instant messaging to output buffer of another user's terminal called SAVED
1965, Aug - mail command added to CTSS in Bulletin 88
1966 - AUTODIN military messaging deployed by Philco-Ford (4 years in development)
1967 - Lawrence Roberts of Lincoln Laboratory now running IPTO at ARPA defines basic goals and starts to lay the groundwork to hire a contractor to begin building ARPANET? Discovers Barans untested message block ideas at Air Force
1967 - SDC's Q32 had a messaging system despite its creator's paper from 1963s claim to the contrary and was demoed to Tom Van Vlek around this time when he visited using DIAL
1968 - Roberts released RFP with packet switching and network data analysis (ala Kleinrock) for networking labs and universities together
1968, Dec - Ted Kennedy's office awards to BBN (Frank Heart) which would link 4 nodes: UCLA, Stanford Research Institute in Menlo Park, UC at Santa Barbara, U of Utah in 8 months by the end of 1969. Willy Crowther writes software for packet switching network. IMP (interface message processor) spec developed
1969, summer - mail command in multics as direct reimplentation of CTSS by Tom Van Vleck
1969 - Official inclusion of inter-user messaging feature (.SAVED) in CTSS release at MIT by Bob Fenichel
1969 - Vinton Cerf captain of crew to install at UCLA, October at Stanford 350 miles away. The first test.
1969 - ARPANET created as 4 nodes on west coast
1970 - BBN is connected to ARPANET as 5th node and sole east coast member
1971 - First network mail - Ray Tomlinson at BBN on TENEX while working on SNDMSG used CPYNET and created the @ sign
1971 - MIT's CTSS system had 1000+ users
1971 - an anti-war screed sent to all users of CTSS is possibly first SPAM using Mail command
1971 - ARPANET has 19 institutions connected
1972 - most machines on ARPANET were running TENEX so Ray Tomlinson's SNDMSG and READMAIL programs as well as CPYNET meant Abhay Bhushan's work on file transfer could also work so Mail and MLFL brought email to ARPANET
1973, March - Xerox Alta, an OS with a GUI released
1973 - RFC 561 to standardize Mail headers (FROM, DATE, SUBJECT, at HOST, etc)
1973 - a study at ARPA reveals three quarters of all traffic on ARPANET was email Because there was so much mail, Roberts writes mail manager software RD
1974 - Vint Cerf and Bob Kahn release TCP IEEE specification
1975 - John Vittal's MSG is a variant of RD mail manager software Steve Walker at ARPA proposes a message group to start work on standardizing mail headers because there are so many different mail manager programs being developed -> first mailing list MsgGrp
1976 - Will Crowther writes Adventure
1976 - commercial email packages begin to appear
1977 - RFC 733 standard for Arpanet messages https://tools.ietf.org/html/rfc733
1978, May 3 - infamous DEC SPAM (though not called that until 15 years later) by a DEC salesman, Gary Thuerk announcing the DEC-20. Richard Stallman defended the sender!
1978-1979: EMAIL developed by Shiva Ayyadurai, 14 at University of Medicine and Dentistry of New Jersey under direction of Dr Leslie Michelson (formerly of BNL). While the development of the software is not in dispute, Ayyadurai makes additional controversial claims regarding the importance of his contribution.
1979 - about 2000 Xerox Altos in use at labs and universities. In December Steve Jobs tours Xerox and sees WYSIWYG mouse controlled interface and soon after incorporates into Lisa, then Macintosh
1979 - Kevin McKenzie a MsgGrp member proposes emoticons :)
1981 - ARPANET had 213 nodes
1981 - Xerox GUI Laurel email application manual:
1982 - Shiva Ayyadurai submits for a copyright on his EMAIL
1983 - ARPANET had 562 nodes, government separates off MILNET
1989 - private providers far outstripped government and ARPANET decommissioned
1999 - Al Gore tells CNN's Wolf Blitzer: "During my service in the U.S. Congress, I took the initiative in creating the Internet." Despite years of misquoting and deriding him in media, Al Gore did spearhead a national policy to transfer defense-funded computer research to the public and to promote universal access.