This parameter is often overlooked and it is poorly documented and generally assumed that this would be set by default. The reality is that it is off by default and can only be set by manually adding the notes.ini parameter. (see technote: https://www-10.lotus.com/ldd/dominowiki.nsf/dx/routerfallbacknontls )
For clarity, this should be added to the server doc next to the other SSL / TLS settings on the Server doc - Ports - Internet Ports - Mail tab, SMTP Outbound.
Here is a related idea too: https://domino.ideas.aha.io/ideas/DOMINO-I-499