The reason why this happened is instructive. Fighting spam is really hard and the best way to do it is to be good at machine learning and to have access to a good percentage of worldwide mail, unencrypted.

Since there's no reason to think that Mastodon is immune to the spam issue, this strongly suggests that whatever replaces Twitter will quickly consolidate to a single dominant player, or perhaps a handful. Or it could collapse under the spam issue like Usenet did.

Not everywhere is America. Most institutions in European countries run their own e-mail servers and deliverability is OK.