Authentication error with Outlook

don't close the window until you vomplete the authentication in the other window. Thunderbird only know about the one it asked to open not the next that outlook send saying close, so i6t continue to attempt to complete the authentication, so just leave it open until outlook stops opening new tabs and windows and then close it.

I left it open and it made no difference; leaving it open or closing it. Uninstalling and deleting all of the things in my original post has made no difference. I can delete and add again other Outlook email accounts but not that one.

The profile file got messed up because none of the accounts on there had any details on the right side of the page when you clicked on the email account under account settings. After I deleted the xulstore file the other account information showed up on the right but the problem account never did or never worked. That was a couple hours ago and things at the beginning are getting fuzzy now. But deleting everything that I deleted and still having the problem with that one but not other Outlook accounts tells me something is stuck somewhere. I even deleted every entry of that email account from the registry and it still didn't help.

Well HO-LEE-COW! After 2 afternoons with Google AI and LOTS of failed AI attempt errors, it found the solution. It was a simple solution and took maybe 5 minutes to fix. I had even created a new outlook account to try and make this thing work, without success. I tried old accounts and 2 worked, others didn't.

What triggered something inside Google AI was when it told me to go into the New Outlook email client and use the mail/forward & IMAP option to let other apps use the IMAP connection. I told it that the option wasn't there. Then AI said to open a new, Private Firefox window. (in Firefox, click on File, then New Private Window. go to https://outlook.live.com/mail/ login to the problem account [CAUTION: There were a couple of attempts to enter the security code sent to the designated alternate email. You have less than a minute to enter before you have to log in again. The good news is that the code sent to the email account is still valid. Have the code ready from the email and enter it as soon as the box appears for you to put in the code.]

Outlook will open the email account and THEN Click on the gear in the upper right corner and click on Mail /Forwarding and IMAP Scroll to the bottom and Click on the Let Devices and apps use IMAP. Bingo! Now go to Thunderbird and add the outlook.com account

It took a lot of effort but I finally got it to work.

Here is AI's explanation: Microsoft completely hid or stripped away the native POP and IMAP switches from the settings menu. The layout you are looking at in New Outlook only shows basic forwarding. This confirms Microsoft has intentionally locked down your brand new account behind its own modern infrastructure, blocking third-party email clients like Thunderbird from establishing a standard IMAP pipeline. You can bypass this restriction by using a web browser to access the advanced backend toggle that Microsoft leaves hidden from the standard app menus.

The Secret Fix: Expose the IMAP Switch in a Web BrowserOpen an InPrivate / Incognito browser window (this prevents it from picking up cached credentials from your older working accounts).Go directly to Outlook.com and log into the new trouble account.Look at the top right corner and click the Gear Icon (Settings).Go to Mail > Forwarding.If it looks exactly like the app version, click the Sign In or Verify Identity prompt that often appears at the very top of that specific submenu page. Microsoft forces a secondary security check here; until you do this step, the rest of the page remains invisible.Once verified, the page will refresh and display the hidden POP and IMAP settings section further down the page.Toggle "Let devices and apps use IMAP" to ON and click Save.

A Warning About New Microsoft Security Defaults. If you flip that switch and Thunderbird still fails to authenticate the new account, Microsoft’s automated system is flagging the sudden connection attempt from an unverified third-party app on your PC.While still logged into the web browser for that new email, check your activity logs to clear it:Go directly to account.live.com/activity.Look at the Recent Activity list for a blocked session that matches the timestamp of your failed Thunderbird attempt.Click to expand it and select "This was me" to officially whitelist Thunderbird on your PC

Once you tell Microsoft's backend that IMAP is allowed and your computer is trusted, head back over to Thunderbird, configure it manually with OAuth2, and it will build your folders immediately.