Forum Discussion
Basic Authentication will end on October so I decided to make a guide on how to handle it
Many thanks for this help. 
It begins to be clearer but still murky. Still better than Microsoft """explanations""".
So now, I followed your process and discovered that three users (myself included) and an automatic mailbox use the simple authentication. I see it is with "Office 365 Exchange Online". And now what ? I can't manage to understand what is wrong and how to fix it.
Thanks for any more explanations.
edit :
I think this is this point I do not understand :
"After providing instructions to users on how to migrate their devices to Modern Authentication your tenant should prevent those users from adding new devices through legacy authentication.". The tenant (me) does not know how to migrate the devices and not even what devices 
Make sure on your Azure AD sign in logs you enable the Columns Operating System and Device Browser used for the sign in, it will help you understand from which software/device the legacy protocal is being used.
In the OS what does it say ? Are you using iOS ? if so then try removing your account and re-adding it because it still might be using Exchange Active Sync.
Forgot, on our website we have a Web App which will tell you what devices users are using and will send you either a daily or weekly reminder of new devices users signed in still on legacy protocols.
More info on a Reddit post
Many thanks for your useful advices.
I would have loved to use the web app but it seems that my M365 business licence is not expensive enough to allow it.
As, on the Microsoft Azure log, the only people to have been signaled with basic authentication (me included) are the only people still with Office 2013, I have tested on my own PC the register modification forcing to use Modern Authentication : HKEY_CURRENT_USER\Software\Microsoft\Exchange\AlwaysUseMSOAuthForAutoDiscover value set to 1 (for the little story behind it, Office 2013 is the last version to NOT have it set by default).
I see now that I am no longer on the "shame list". I'll watch for a while and if my only problems come from it, I'll feel lucky.
Thanks again.
