IPSharp had an Email system that operated under the AplLanguage.

"The SHARP APL electronic mail system for OS/390 was originally developed in the early-1970's and used extensively by its originator, I.P.Sharp Associates. Mailbox continues to be in use today." (from http://www.soliton.com/Systems/systems.html) Soliton ( http://www.soliton.com/aboutus.html ) is the successor to I. P. Sharp Associates that resulted from the acquisition by Reuters in 1987.

''The '666 mailbox' function was implemented using a relatively sophisticated file subsytem, running on an IBM 360/50. When I got there in 1972, it had been operational for at least a couple of years due to the efforts of many people who are still technically active including: LeslieGoldsmith, ... -- Hwo.''

ARTEX used IPSharp Mailbox: http://alien.mur.at/rax/ARTEX/

1971: The first Arpanet email
 http://www.olografix.org/gubi/estate/libri/wizards/email.html
 http://www.mailmsg.com/history.htm

Non-network email, available only to users of a particular mainframe, was invented even earlier, in the 1960s.
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Q: We are talking about 1972 here. E-mail in 1972? How did it work? 

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I suspect IBM's VNET (RSCS) dates from around that time, too.  The story goes that, when IBM management first heard about it, the guys at Research had already linked together 200 computers at various locations. One of the managers dropped his coffee! --PaulMorrison
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Re: "How did it work?" - I.P. Sharp operated a commercial timesharing system. Customers were linked together via a proprietary network, which eventually became X25-based. Mail resided on the central servers (mainframes). A fairly sophisticated system of user group/list management, mail status (including receipts, importance levels, confidentiality indicators, etc) was available. Originally this system was only available on the IPSA network from their servers, but by the late 70's it was sold to numerous large multinational companies who hosted their own systems (usually but not always on the IPSANET). I don't have exact figures but estimate the system had a few thousand users by 1974, rising to perhaps 10-20K in the eighties. -- Clement Kent

''Hi Clement!  from HansWobbe''.