New opt-in endpoint available for SMTP AUTH clients still needing legacy TLS

Thanks for that. Quite a few ways to set this up it seems. Hardcoding is another option if all fails; something we've seen commonly with say Paypal IPN. Not the recommended practice but a practical solution if you need one. Useful if you can't upgrade .NET or apply registry changes.

We're ready to go on our end; though still a few unanswered questions and greviances:
A) Just out of curiousity sakes, has anyone actually ever gotten this supposed error, where was it supposed to be read:

421 4.7.66 TLS 1.0 and 1.1 are not supported. Please upgrade/update your client to support TLS 1.2. Visit https://aka.ms/smtp_auth_tls.

We tested Telnet, OpenSSL, Powershell .net smtpclient and the live site .net smtpclient and neither error we recevied was like this at all. The account in question doesn't get this error since it's rejected before it even gets to that point. The error we saw was simply a generic "Failure sending email" so it took us a while to get to the bottom of the issue of our own accord.

B) Still need a timeline or an update on when this is expected to go live. Would also be nice to have some transparency to understand the schedule and intensity of the "Speedbump". Microsoft are aware this is having a tangible impact on the livelihood of businesses and clients' live sites, correct? So it'd be nice to know what to expect from this "Speedbump" beforehand, is it 20% of all emails, is it more, is it running on a timeframe? Why did we have 2 months of no issues for instance?