Outlook 2016 doesn't accept password or app password

Same problem with iOS 17.3 Mail app on iPhone and several Win10 and Win11 machines with Outlook 2019.

I created the registry key you suggested, but still the problem persists. I am on Office 365 running Windows 11 with all the latest updates.

The username/password combo is not wrong, if you press OK a few times on the error box that appears, it will finally login, only to randomly fail again later.

The problem appeared sometime within the last 10-15 days, at first it was happening with the SMTP outlook server, the problem with the SMTP server resolved itself after a few days but now same problem happens with the IMAP outlook server.

I've been troubleshooting it for at least 12 hours, also spent 2 hours last night with Microsoft level-1 tech support through Quick Assist, they couldn't find a solution so they escalated it to L2 but I'm sure they won't be of any help either because I am almost sure it's a Microsoft problem. Something must have changed on the IMAP server and/or the end-user client (Windows PC) through windows updates (something that probably has to do with authorization), and the problem started appearing.

It was also a problem with Thunderbird, and it seems to have been fixed when the user enabled Oath2 authorization in Thunderbird. I was also experiencing the problem with Thunderbird and enabling Oath2 authorization fixed it ! There's no such option in Outlook for windows.

Just posting this in case MS techs need further info on troubleshooting the problem. Keep in mind that the Outlook app for android and the outlook webmail work without problems. I'll just stick to Thunderbird for the time being, until this is resolved.

I think it originated from same source as MS 365 outage and once they will figure out what was done to it, it will be resolved. I have same issue with Office Outlook 2013 on 3 different machines and probably on 4th too, but none with Windows Mail app. Which is horrible substitution for Outlook, but it works as well as webpage which now mostly fully functional. It went to point of some things not working in web Outlook as well. Issue is not on our end but on Miscrosoft's. 365 installation took it better, but exhibited some other glitches.

Since Tuesday night (23Jan) I've had connections refused to Outlook.com accounts set up long ago on my Android phone using Exchange ActiveSync. This has been on both the GMail and BlackBerry Hub apps. Both report that I need to update my [app] password: of course I don't do that as it hasn't changed and attempting to do so will certainly break it as there is no server connection to accept a new password.

There seem to be occasional sessions at long intervals where something gets through but nothing is prompt.

Are EAS connections to Outlook.com still allowed? So many functions are being "deprecated" with no effective warning that I suspect that to have happened.

Outlook 2019 is having the same problem. The Mail app on iPhone still works for my account.