In article <a1f45fdbf44300cc0e6058b3e52568f3d0a61091.camel@interlinx.bc.ca> you write:
It's a real pity that there appears to be no real-world use/implementation of RFC8689.
I implemented RFC8689 as soon as Jim proposed it. My MTA recognizes the REQUIRETLS option and then ignores it. A lot of people who really should know better imagine that they can announce something on the Internet and other people will have to do what they say. It has never been true, and it is still not true. We've seen this before with SPF -all where people are surprised that other mail systems accept mail anyway. Opportunistic TLS is fine, as is MTA-STS which says "if it doesn't offer STARTTLS it's not me". Neither of those purport to tell other systems what to do. R's, John