At the time, most email was either local to the host (big mainframe in the basement) or transferred once a day through scheduled dial-up connections during off-peak phone hours

Which is a good fit to a distributed, offline first version control system, if you use it like that.

I saw an application once about 25 years ago that was built on SMTP for communication. I think it was an inter library loan system, but can't remember for sure. I was a pretty smart way of not having to worry much about redundancy in the communications.