Basic Authentication and Exchange Online – September 2021 Update

Wolfman92  - first off, well done for all that proactive work getting the vast majority off basic. 

Sadly for you in this case, if you missed the opt out window, you will have to wait for us to turn off basic auth before you can re-enable it. 

I strongly recommend using Auth Policies to create a group of accounts you know need basic for a specific protocol, and assigning it to them. Then setting the default policy to block. Then, when you re-enable the protocol, only those that you know need to use it, can still use it. 

But in short - if you missed the opt out, you will have an outage. 

Oh, and the latest blog - https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-deprecation-in-exchange-online-september/ba-p/3609437 is the one to watch. This blog is a year old.