Could JMAP replace SMTP as well as IMAP?
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.
Can I know from a capability exchange if the remote server will send emails after adding to the "Outbox" folder?