I don't understand the complexity of Microsofts implementation which they call modern authentication So I can write nothing. I think you will find a similar issue for most folk in the open source world, that really do not have much association with Microsoft proprietary technologies. Yes, oauth is open source. Modern Authentication is not and it comes with scopes and other Microsoft special features. Then they change it all the time in the hope folk like you will give up. The simple answer is it works for me on my old hotmail accounts without issue and it will work without issue for most folks that do not use a third party antivirus or a VPN just as well as it work for me. I do however have to use an SMTP server other the the one Microsoft publishes (smtp.office365.com). Those that have antivirus products that block connections, privacy tools that turn of cookies and VPNs which Microsoft tends to block because the endpoint is a long way from where the phone you use with the same Microsoft account is located dfo have issues caused by their software, not Thunderbird. Those that have a managed office365 account where there is an admin allowed to specify an almost unlimited number of settings, including what software and what versions of that software are allowed to use Modern Authentication are frequently going to have issue. Bottom line with this one is the admin needs to be in a position to offer user support if they are changing the defaults. That is not something that can be done by anyone but the organization offering oauth as no one else knows what requirements they have made. Now do you actually have a question about your Thunderbird setup? Or is this just a misdirected feedback that should have been posted here https://connect.mozilla.org The location is linked from the Thunderbird help menu as ideas and feedback.