This has already happened! My school promised email for life, and uses Microsoft's Office365. A couple months ago they sent out emails saying if you don't update your client to use "modern auth" (which was a new term for me) I'd lose the ability to check my email.

Well Microsoft blocks the Thunderbird embedded browser, so you can't complete the Oauth2 login. I've been effectively locked out of my account since (I'm not going to use the webmail just for this account - set up a forwarding email and told everyone to use my new address).

Microsoft doesn't by default block the embedded Thunderbird browser. An admin can however enable that setting.

Guess it would be nice with a fall-back option in Thunderbird to use the system browser.

So spoof the user agent.

That letter isn't from microsoft though, I manage several tenants, and none of our end users or admin got that letter.

specifically what is missing, is that email using basic auth won't disconnected OCT 1st because of the tons and tons of machine that do things like email faxes, and scans that do not support modern authentication.

This looks like whoever "enterprise services" is, they wrote a brief message that wasn't very accurate.

We split off any of our "legacy" devices to SMTP2GO accounts.

It gets worse.

"We won't support clients that dont support microsoft supported practices" is a very short stone's throw away from "we won't support non-microsoft clients", as I've come to discover at my university, where I had to ask for permission to use an email client just for oauth to work.

Which is yet to be granted after 2 months (and if granted, will be granted site-wide ... i.e. unlikely unless there is a large demand for non-microsoft clients)