What play-by-mail (or email) RPG had the largest number of players?
I've recently been looking into "Net Games", play-by-post (not email) RPGs run by a company called Youentai in Japan in the 90s that seem to have peaked around ten thousand players, and was wondering if anything was larger. Looking around I could find a few English-language games, like Saturnalia, that claim to have had about 3000 players at one point, but nothing larger.
Because of the age and niche status of these games information is often not online, and I know any numbers will generally be unsubstantiated, but I'm curious about the history of large games.
To clarify: by "largest game" I don't mean a network where there are, for example, ten thousand people playing with the same rules but different sessions (like a network of chess players), but rather the largest number of people in a single shared world where all characters could (at least in theory) interact, much like a modern MMO server.