JMAP was developed by the guys that run FastMail (who are the primary developers of the open source email server Cyrus-IMAP). It’s easier to implement and more performant.
Side note, Cyrus is a pretty amazing mail server. It doesn’t get much love here, but it’s bombproof, fast, supports multi-node clusters, IMAP, NNTP, CalDAV, CardDAV and more that I’m forgetting. It’s just a bit old school as far as configuring goes.
I think there’s a lot of FUD around this. Yes, deliverability can be a PITA, but with a clean IP and good setup it’s usually solvable. Worst case, you can pay a small amount to use a 3rd party SMTP relay and still get most of the benefits of selfhosting. It wasn’t deliverability that made me stop selfhosting it was spam, and it wasn’t that dealing with spam was that hard, it was just annoying.
Miniflux?