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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You don't need major providers to support it, they support SMTP and that's how messages are relayed. JMAP is just so you: the client, can fetch your mail from wherever you host your mail.