Last time I looked at IMAP, I was a little peeved that servers didn't accept messages added to the "Outbox" folder and send them, instead requiring that clients talk SMTP and then (or not) save a copy of the same message to the "Sent" folder.
Storing sent mails is really one of the oldest problems of the SMTP/IMAP combination.
BURL [1] is an SMTP extension that allows to submit a mail that is already stored on an IMAP server. This still requires to make a separate connection to send the mail, but at least it does not require uploading it to both SMTP and IMAP only to store it in the Sent folder.
However, this extension is rarely implemented or deployed, although the RFC itself is already twelve years old. As an example, Dovecot started to support it by acting as a SMTP proxy [2] as of version 2.3 in 2017. I am not aware of any mail clients that support BURL.
I checked out the git source and built it from there. It was non-trivial to get it built and configured properly for JMAP, but I wanted the latest as the spec is still in flux.
No, not as far as I'm aware. But it's an idea I've also had. Upload the message to drafts, and then when you're ready to send, just move it to Outbox right?
Could JMAP replace SMTP as well as IMAP?