I had this problem, it started unexpectedly at around 18:00 UTC yesterday (6th July 2022). It affected two different Thuderbird installations, one PC was running Win 7, another was a Win10 laptop at a different location and up to date. It seemed fairly certain it was a problem with authentication on the mail server side which is provided by Microsoft as part of an Office 365 sub ... & now, confusingly, called 'Outlook'. (I know, but it was the way our ISP wanted to provide our email service and I didn't have time to argue the toss or learn how to run a mail server as well as everything else e.g. learning Linux, as I am now desparate to get away from MS & Windows).

This morning the Win 10 version was intermittently working, then it would sudden go back to showing the error message > "Login to server outlook.office365.com with username xxxxx@xxxxxxxxx failed." Sometimes on entering the password anew, it would start working again. Then it seemed to have stopped working again completely. How odd is all of that? I'm thinking this probably could be related to a SSL security certificate issue in the Microsoft mail servers, where some servers have expired certs and others don't ... which would explain the erratic behaviour, at least.

The older Win7 machine here still seemed absolutely broken, no amount of trying seemed to work.

Thunderbird's help forums proved very helpful however:

> tim193 > tim193 Question owner > 6/17/22, 6:48 AM > > Issue Sorted - Changed Authentication message from "Normal Password" to OAUTH2 and connection now running normally

Exactly in the area where I suspected the problem would be. I did the same and everything sprang back to life. Firstly got a new Dialog box from Microsoft to allow privileges for the mail client, which had to be OK'd of course. Now all sorted & working normally again.

I'd LOVE to know exactly what change to the Microsoft servers caused this, but we will probably never find out. Of course for less technical users they will just think Thunderbird is broken or unreliable ... oh, how convenient for Microsoft, sweep everyone into the dubiously loving arms of Outlook. Er, No Thanks MS! You can Foxtrot right Oscar.

I had this problem, it started unexpectedly at around 18:00 UTC yesterday (6th July 2022). It affected two different Thuderbird installations, one PC was running Win 7, another was a Win10 laptop at a different location and up to date. It seemed fairly certain it was a problem with authentication on the mail server side which is provided by Microsoft as part of an Office 365 sub ... & now, confusingly, called 'Outlook'. (I know, but it was the way our ISP wanted to provide our email service and I didn't have time to argue the toss or learn how to run a mail server as well as everything else e.g. learning Linux, as I am now desparate to get away from MS & Windows). This morning the Win 10 version was intermittently working, then it would sudden go back to showing the error message > "Login to server outlook.office365.com with username xxxxx@xxxxxxxxx failed." Sometimes on entering the password anew, it would start working again. Then it seemed to have stopped working again completely. How odd is all of that? I'm thinking this probably could be related to a SSL security certificate issue in the Microsoft mail servers, where some servers have expired certs and others don't ... which would explain the erratic behaviour, at least. The older Win7 machine here still seemed absolutely broken, no amount of trying seemed to work. Thunderbird's help forums proved very helpful however: > tim193 > tim193 Question owner > 6/17/22, 6:48 AM > > Issue Sorted - Changed Authentication message from "Normal Password" to OAUTH2 and connection now running normally Exactly in the area where I suspected the problem would be. I did the same and everything sprang back to life. Firstly got a new Dialog box from Microsoft to allow privileges for the mail client, which had to be OK'd of course. Now all sorted & working normally again. I'd LOVE to know exactly what change to the Microsoft servers caused this, but we will probably never find out. Of course for less technical users they will just think Thunderbird is broken or unreliable ... oh, how convenient for Microsoft, sweep everyone into the dubiously loving arms of Outlook. Er, No Thanks MS! You can Foxtrot right Oscar.

While using Oauth2 seems like its registred an app under the user in tenant ..

It is unclear whether you are still having problems. Yes, switching to Oauth is the right direction. Unfortunate that your email host didn't apprise you of this.

It is unclear whether you are still having problems. Yes, switching to Oauth is the right direction. Unfortunate that your email host didn't apprise you of this.

This addon is also doing the job

https://addons.thunderbird.net/en-US/thunderbird/addon/owl-for-exchange/

I finally got my live account to start getting email with leaving the Normal Password setting. I had to go to recent activity for this accent and say it was actually me and then my email started again.