From the book Positioning (which is OK):
Electronic mail
Developed jointly with the U.S. Postal Service and inaugurated on a limited experimental basis in 1970, Mailgram is the nation's first electronic mail.
To send a Mailgram, you call Western Union, which transmits the message electronically to a post office near the recipient. The Mailgram is delivered the next business day.
In addition to sending Mailgrams by telephone, the customer can also send them by telex, TWX, magnetic tape, computer, facsimile equipment, or communicating typewriters.