Thee most commonly used network service is electronic mail (E-mail), or simply as mail.

Telegraphs have been largely replaced by other forms of telecommunications, such as fax machines and electronic mail (E-mail), but they are still used in some parts of the world to send messages.

That file was electronic message, so suddenly in about 1971, we had e-mail. So an e-mail was pretty much the major use of the internet for the next 20 years.