JMAP is a great initiative from FastMail to develop, open, and spread their modern successor to IMAP. This is badly needed — I hope it catches on with client and server developers.
The good news for such an effort is that there are very few influential email hosts left (and FastMail is one), and even fewer email clients. The bad news is that it’s very hard to get the remaining few to do anything.
Fastmail is our email provider here at Data McFly, and we've been happy with them.