Has any other game ever done multiplayer via email? Civ 4 had an option where you could play by emailing the save back and forth.
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Goo Truther 
@fillertrack https://deepwiki.com/freeciv/freeciv-web/5.2-play-by-email-(pbem)
https://en.wikipedia.org/wiki/Correspondence_chess
and a bunch of other old play by email games were also 4X games but there isn't much of a reliable list.
This site organizes older style play by email games as well, with its own list http://www.gamesbyemail.com/
Conceptually, games which play by shared save file synchronization work similarly. So things like Unciv which use DropBox are kinda the same thing.
