Here's a kind of "outside the box" thought.
Why not extend the IMAP protocol to send email? Instead of having to use SMTP if an addition were made to the IMAP protocol then IMAP could transport outgoing mail to the server the same way it moves messages to the server. On the server end Dovecot would deliver outgoing email to the host MTA via localhost:25.
The advantage would be that clients need not have to deal with a separate configuration for outgoing email. If they have an IMAP connection and already authenticated and perhaps encrypted, then they can transport outgoing mail over the same connection.
Of course - email clients would have to have this feature added as well, but it should be trivial. It could be as easy as moving a message to an "out" box on the server, but the out box really is outgoing email rather than a real imap folder.
Am I the first one to think of this?