R> I am learning Dovecot step by step. I have enabled the Submission R> Server, in the hope that I would not need to learn other MTAs like R> Postfix.
It's not that hard to setup postfix to accept incoming email from the internet and your local users, and to then pass it to dovecot as needed. Dovecot is for IMAP/POP (shudder) access of your stored emails. It's not for sending email, or even receiving it from the outside world.
R> The Submission Server is very comfortable: it picks up the existing R> Dovecot configuration, so that you do not need to configure any R> user authentication separately. It is working fine on my test R> setup.
It's not hard to setup postfix/dovecot to use the same authentication. My system used plain files. Trivial.
R> My first thought was that, if the recipient is a local mailbox, the R> Submission Server would not need to relay the message to any R> external SMTP server, as it could just deliver it locally. After R> all, it is running on the same Dovecot.
Really, you're trying to optimize the wrong thing. Just setup a linode (or anything else except digital ocean since charter.net blocks them completely for email delivery) at $5/month and install postfix/dovecot together. Works great.
John