The first version of a computer email system that resembles anything like what we have today showed up around 1965. Computers had been primitively networked a few years earlier, and you could technically send and receive content.
At MIT in 1965, Tom Van Vleck and Noel Morris wrote the mail command for the Compatible Time-Sharing System (CTSS) which allowed users to send mail (a file) to each other. When the recipient logged in, they would be notified and could print the contents of the email. You can read the info segement ("man page") here.
In CTSS, each user's MAIL BOX file was "private" mode. Only the user could read or delete PRIVATE mode files. The MAIL command was a "privileged" command that could write into another user's PRIVATE mode files.
-- Tom Van Vleck
In 1967, "messaging" was on the list of the reasons for developing ARPANET, the precursor to the Internet. The other reasons included: load sharing, data sharing, program sharing, and remote logins.
In 1969, Tom Van Vleck would reimplement the mail command for Multics.
In Multics, we did not have "private" mode files, and so the initial mail command appended mail messages to a world-writeable mailbox file in the user's home directory. We knew that this was unsatisfactory, and a few years later, replaced this version of mail with a facility that moved the updating of mailbox files to an inner "ring" of execution, that provided "extended access control" flags a d r o s w in access control list entries. This enabled a user to set the mailbox's access control list to a list of ACL entries that allowed
a) the owner to read, write, and delete messages in his or her mailbox freely
b) give permission to specific users or to all users, to append messages the mailbox
c) give permission to users to delete messages they created
d) prevent non-owners from finding out anything else about the mailbox's contents
-- Tom Van Vleck
The U.S. Postal Service experimented with forms of electronic mail in the 1960s and 1970s. An effort called MAILGRAM between USPS and Western Union sent messages electronically to a post office where they were printed out and mailed as normal letters.
In 1971, the SNDMSG and READMAIL programs for TENEX allowed users to specify the traditional Subject:, To:, and cc: headers. It also introduced the @ sign to refer to an ARPANET host.
The same year, a mail command was added to Unix (another copy of Tom Van Vleck's implementation for Multics and CTSS). The Mail Box Protocol (RFC 221) was designed.
In 1974, MSG was the first mail application that included features like Reply and Forward. The Unix MBOX format was developed. Attachments came in 1976.
Later:
- 1982 – Simple Mail Transfer Protocol (SMTP)
- 1988 – IMAP
- 1991 – PGP for Email
- 1995 – First commercial webmail company
You can see the entire timeline until 2011 here.