MCI International launched its Telex World Letter service in about 1986
The Telex World Letter trademark was filed by MCI in 1986, which may have been when the service was launched.
Source: Justia Trademarks
Image: Public Domain via Wikemedia

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I remember using this service as a telex operator in the late 1980s. The Telex World Letter was an innovative way of sending a letter from a telex machine to be delivered anywhere in the world, free of charge, courtesy of MCI International.
The cost to the telex subscriber depended on the price of a telex connection to MCI International in the USA.
How was a Telex World Letter sent?
The telex operator would connect to MCI International’s server in the USA by calling telex number 666400. Upon receiving the MCI TELEX WORLD LETTER answerback, the originating telex machine’s answerback was automatically taken by MCI’s server.
Thereafter, as the operator was communicating with a computer, strict message formatting rules applied for sending the message. This formatting can be seen in MCI International’s instructions (adapted by me) below.
An example of how a Telex World Letter would have been formatted
System Prompt
Operator Response
MCIWORLD LETTER
YOUR ANSWERBACK
IS HELP DESIRED?
NO (CR)
LETTER NUMBER: 100/2
(system generated)
TO:
JOHN SMITH (CR) (LF)
COUNTRY OF DESTINATION
ENGLAND (CR)
ADDRESS LINE 1:
NAME OF COMPANY (CR) (LF)
ADDRESS LINE 2:
022 FLEET STREET (CR) (LF)
CITY:
LONDON (CR)
**STATE: (OR PROVINCE:)
**ZIP CODE: (OR POSTAL CODE:)
EC4Y 000 (CR)
YOUR NAME:
JOHN SMITH (CR)
YOUR COUNTRY:
USA (CR)
YOUR TELEX NUMBER:
000000 (CR)
START OF MESSAGE:
YOUR MESSAGE (CR)
END OF MESSAGE SIGNAL:
MMMM (CR) (LF)
Further input options were available. For instance, it was possible to send multiple messages during the same connection. For more information, view the source document and click or touch the link entitled DOWNLOAD PDF.
CR is short for carriage return. LF is short forline feed. Telex machines sent and received messages using capitalised letters.
Further reading: View an MCI International advertisement for the Telex World Letter. PDF file.
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