So reading this, this attempts to standardize what I understood to be best practice last time I ran a mail server - TLS required for submission and access. I see this as a good thing.

But this doesn't seem to touch on transmission between email servers at all, and that needs to be addressed as well. Email can be submitted protected by TLS, but if it has to cross the Internet in the clear to reach its recipient, then all you've succeeded in doing with submission/access TLS is prevented your user's login credentials from leaking. You've not done much to protect the confidentiality of your user's correspondence. Understand, I'm not denigrating the importance of TLS for submission and access, but it's also not the whole picture.

Yes, you can set your SMTP server to opportunistically use TLS when the other connecting SMTP server supports it as well, but not all SMTP servers do. Mandating TLS makes those servers that don't effectively unreachable. I seem to recall Facebook doing a study about it. The Postfix documentation warns about it as well.

I'm not sure I have a good solution here, other than something akin to the linked RFC: a proposed standard that gives a deprecation/sunset period to transition to mandatory TLS. The problem then, is getting majority compliance during that period. If the IPv6 transition is any indication, that's likely to be an uphill battle, as mandatory TLS between SMTP servers would break a rather large installed base that's presently working just fine.