[craig_s_bell]

You might be thinking of Zephyr, which was part of Project Athena. My school was using Athena, and I occasionally received zephyrgrams from people at MIT.

http://web.mit.edu/sipb/doc/working/izephyr/html/izephyr.htm... https://en.wikipedia.org/wiki/Zephyr_(protocol)

[amichal]

That was it! Thank you for the references

[dsr_]

The Zulip chat system is explicitly based on the same ideas, if not the same code.