Civilization V has a "cloud game" option that keeps the save file online. Players can then take their turn when they have a chance (though non-responsive players can break the game), and if you're running the game through Steam you'll even get a Steam notification when it's your turn. You can also define a webhook that gets called when it's somebody's turn. One of these days I'll put together a little tool that takes that webhook message and translates into a Discord PM or Discord channel post @-ing the person whose turn it is.

They specifically say that it's their way of paying tribute to Civ playing by email.