That centralization mostly occurred due to the economics of below-cost pricing though. It's not necessarily due to federation itself failing as a mechanism for

That centralization mostly occurred due to the economics of below-cost pricing though. It's not necessarily due to federation itself failing as a mechanism for email. In other words, Gmail is the most popular email provider because it's in Google's interest to have people be logged in all the time so they can personalize search results, so that incentivizes them to make a very good email system and then give it away for free. Giving something good away for free because you gain indirect benefits via some other business will rapidly centralize more or less anything, which is why there are at least theoretically rules against market dumping and tying (which aren't really enforced in the software world, but that's another matter). We can imagine a parallel universe in which search is far more competitive, with lower margins, and thus Google couldn't financially justify subsidizing consumer Gmail for so many years. In such a world it's likely that there'd be more players, perhaps they wouldn't be as good but there'd likely be more of them, there'd probably be companies that specialized in selling spam filtering tech to them and it'd look more federated than what we have today.