Here was the first three hours of my morning (text below was taken from an email I sent everyone after 'fixing' the issue)
Microsoft is discontinuing old methods of how Outlook connects to the cloud email server (“basic authentication” is being “retired”). We use Outlook 2016 which, by default, uses “modern authentication” so it shouldn’t have affected us……
After trying to figure out what else could be the problem, I finally determined that most of our Outlook 2016’s are in fact using basic authentication (which is what Microsoft shut off yesterday).
Then I found Microsoft has a “turn basic authentication back on until the end of the year” (stay-of-execution) feature. I turned it back on at 10:05am and people started getting email again about 20 minutes later.
Moving forward, we’ll need to get everyone’s Outlook using modern authentication or the same thing will happen again at the end of year – and it will be permanent. I’m also curious why 3 users didn’t experience any down time this morning. We’re on it!