alt Hacker NewsFastmail was developing JMAP for a while, it's not gotten a lot of uptake (mostly because Fastmails primary mail partners like Gmail, AOL, Yahoo and Hotmail are all 20 year old legacy dot com companies...did Microsoft spend all day trying to get Fairchild Semiconductor to play nice with them? No, they did a worse is better DOS and the rest is history).
But email is a least common denominator, and like how plan9 failed to take over from unix bc unix inertia, JMAP or deltachat IM over email won't take over bc of network effect inertia, I suspect.
> mostly because Fastmails primary mail partners like Gmail, AOL, Yahoo and Hotmail are all 20 year old legacy dot com companies
Pretty sure it's because MS wants you to use Outlook New (New) New and Google wants you to use their web interface. Nothing to with them being old, but everything to do with owning as much of your data as possible and have as many opportunities to show you ads as possible.