Vern van Vlear added a crude mail facility to the 940 exec early on. Mail was sent by a teletype input such as:
MAIL SUSAN
LETS HAVE LUNCH TODAY.
“SUSAN” was the name of another user on the same 940. The next time the Exec was talking to Susan, she would see:
MAIL WAITING
The command:
MAIL
would elicit:
FROM TOM:
LETS HAVE LUNCH TODAY.
There were commands to delete mail or to see if it had been delivered. There was even a command to unsend undelivered mail. There was no mail between machines. There was one 940 that Tymshare developers used to exchange mail. Other operating systems had similar features including, I think, CTSS.

We told a few of our customers who were geographically distributed about this and they used it, especially to talk about computer things but otherwise too.

Shortly after we installed a trans Atlantic Tymnet link this became sensitive. I heard the following: At some embassy party in Mexico City one of our customers told an AT&T rep: “We don’t have to pay your exorbitant telegram tariffs: we just send mail on Tymshare computers. Long story short—we told our customers not to talk about mail unnecessarily.

When we were negotiating about service in Japan a few years later the Japanese were savvy and wanted us to put quarantines on files so that a file written by a user in Japan could not be read by another not in Japan, and the other way round. We did not do that and Japan service was delayed.