We need better client support for JMAP. Apple Mail, Thunderbird, Outlook (as if), and so on. I'm surprised some of the smaller ones like Canary or Spark don't implement it as a product differentiator.
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I’ve got a friend who’s been pitching me on building a new email client for years. “I’ll do it if we exclusively use JMAP.” “okay does that include Gmail and Apple/iCloud accounts?” “Nope.”
I could sort of see dual-supporting Gmail's proprietary API and JMAP, but unless the #2-5 competitors support it… what’s the point? (sorry to put on the pessimism hat)