I've seen some providers like Gmail already using MTA STS records and subdomains, but they don't read the records of my domains yet. Still it's an interesting solution.
I'm disappointed in the mailing lists software and configuration "in the wild". Refusing to just change From address and put the original From into Reply To and breaking DMARC (that I've seen some operators just claim "is badly designed") is a very sad state of the world.
I've read JMAP spec but was not pleased with the RPC style design. I've seen "why isn't it REST" and the FAQ says REST is not mobile friendly. Still, reading all these Foo/get methods looks bad to my eyes (sorry, personal preference) an I don't see how it could improve caching (as the FAQ claims). I like the idea of replacing IMAP with something modern though.