I think there's some history missing here. "The RFC proposed a split of the traditional message submission and message relay concept." What this is misses is

I think there's some history missing here. "The RFC proposed a split of the traditional message submission and message relay concept." What this is misses is that the former specifications never accounted for message submission in the first place. In the beginning, an MTA was installed on every system, and "sendmail" was used to get email submitted into the MTA. So there was no need for any other protocol to do this. But this only worked on Unix. When less-well Internet-connected hosts came along (eg. Windows), there was a need to be able to submit mail to an MTA that was on a different host, since one was no longer present locally. Lacking a specification for this, MUA implementers just re-used (and overloaded) SMTP, by making the MUA pretend to be an MTA. This caused a mess because MTAs were liberal in what they accepted (Postel's law), so they needed to tidy up badly formed submitted email such as fixing up headers. But this broke MTA to MTA transmission because email flowing that way ended up getting modified in unexpected ways, breaking old assumptions that MTAs generally only added headers. And finally, more recently, the spam problem meant that MTAs needed to authenticate clients requesting relaying, since they didn't want to be labelled as spam originators. Proposing "a split of the traditional message submission and message relay concept" was really just about sorting out the mess that we de-facto ended up in.