As far as I can see, jmap only provides means to upload email to your email provider, it doesn't actually tell how that email ends up from one email provider to another, which is what SMTP does.

Granted it's already nicer for clients not to need to configure SMTP to begin with.

Yes, one confusing aspect of SMTP is that there is a server-server part (listening on port 25) and a client-server part (465 or so, usually authenticated). I haven't dug in-depth, so maybe they are exactly the same protocol though.

Acessing port 25 of server is usually blocked by ISPs as a way to prevent spam.

The difference in ports is due to SSL/TLS expected to be automatically applied on 465 iirc like 80/443 for HTTP(S) (you can also encrypt 25 by issuing STARTTLS after setting up a connection but it's not the default and might fail I think).

Authentication with mail is separate, usually to allow for relaying whilst anyone can usually drop emails IFF your server is the destination.

Confusing and needlessly complex? Yep. Natural result as of uncontrolled evolution? Yep.

One of the big mistakes with the design of IMAP was that there wasn't a standard Out-Box where a client could rely on the IMAP server for sending the email. I'd have loved to see a world where mail clients only had to implement IMAP and nothing else.