Outlook 2016 doesn't accept password or app password

Whatever 9Folders fixed isn't directly related to this issue - let's not use this thread has marketing vehicle for 9Folders.

I think the issue is fixed now by MS, probably coincided with 9Folders' timing. I think MS was messing with server-side authentication related stuff that caused this mess.

The issue occurred on all Microsoft-owned apps, i.e., Outlook for Android, Outlook desktop various versions. It also impacted other email clients such as GMail that used Exchange ActiveSync, Other protocols seemed OK.

Around 2am Pacific this morning, I did the following sequences and the issue is resolved and no more failure or intermittent failure since then. I used most of the steps before since 1/24, but it never lasted until this latest attempt, suggesting something was fixed on the server side.

1. Removed the impacted MS account from email client and OS, i.e., I removed it from Android OS's account setting.
2. I reset the Cache of the email app but didn't clear data, in my case GMail.
3. I rebooted my device just to release any Exchange ActiveSync related components that may have left opened.
4. Here seems to be the important part - go to your impacted email account online via Outlook Web. Go to Settings then search for "mobile", it should take you to a setting page and your logged in Mobile devices are listed. Use the delete function to remove and deprovision the device from your account. This setting is available for both free Hotmail/Outlook account, as well as paid Office 365 business accounts.
5. Reboot again just to be safe.
6. Go through the normal add account steps. I used "manual setting" instead of let it auto discover my email account's server settings. For my personal Hotmail account, I used eas.outlook.com as server address. You should see setup successfull/complete.
7. You will need to resetup your usual settings like sync frequency and such and that's it!

I have been monitoring this since 2am and no intermittent failures like it has been for the last 5+ days.

I have also looked at the security history of my Hotmail account. Prior to this around of mess, every authentication attempt is listed separately. Now, they are grouped! The difference is very clear, everything up to 1/23 is separate and now they are grouped by the device or session. Every ActiveSync sync is listed under one group. Every web access is listed under another.

The best I can tell is that whatever MS fixed, it require a new device instance and session, so you have to go through the delete device process.

Good luck!

Whatever 9Folders fixed isn't directly related to this issue - let's not use this thread has marketing vehicle for 9Folders.

I agree, but no need to tell me, I'm not using the thread to market 9folders, in fact for several days I've been doing the opposite, trying to keep the thread focused on the underlying Microsoft server side issues.

I think the issue is fixed now by MS,

Not yet, in fact as bad as ever on my devices, but each of us may experience this to varying degrees and at varying times as the problem and subsequent solution work their way through the network

I think MS was messing with server-side authentication related stuff that caused this mess.

Similar suspicions here; it may be no coincidence that this occurred as Microsoft is going through a change in their various Outlook client offerings and to your point they may have over-fiddled with something.

The issue occurred on all Microsoft-owned apps, i.e., Outlook for Android, Outlook desktop various versions. It also impacted other email clients such as GMail that used Exchange ActiveSync, Other protocols seemed OK.

Yep, all clients except Microsoft Outlook for Android, and 9folders with their emergency update. Except for 9folders, I verified all of this at my desk and posted some detailed issue reports in previous pages and in a MS ticket.

4. Here seems to be the important part - go to your impacted email account online via Outlook Web. Go to Settings then search for "mobile", it should take you to a setting page and your logged in Mobile devices are listed. Use the delete function to remove and deprovision the device from your account. This setting is available for both free Hotmail/Outlook account, as well as paid Office 365 business accounts

.5) Reboot again just to be safe.

6. Go through the normal add account steps. I used "manual setting" instead of let it auto discover my email account's server settings. ...

Hmmm. I think you are onto something here and have stumbled onto part of Microsoft's proprietary grab. I've never intentionally added any apps to that setting in any of my six affected accounts and now that I check I see that the android Outlook app has been automatically placed itself in that list for all accounts without my interaction. (This list is supposed to be a list of devices but Microsoft has placed an app which exists on multiple devices in the list - good grief). In any event that workaround does not work for me but is worth exploring.

The best I can tell is that whatever MS fixed, it require a new device instance and session, so you have to go through the delete device process.

Good luck!

Actually still not fixed, and affects accounts for which there is no device/app to delete including (Ms Office Pro products) and there seems to be no way to change that at the account level settings or in email settings.

When Microsoft fixes it they will most likely say so here where they announce the issue: https://support.microsoft.com/en-us/office/fixes-or-workarounds-for-recent-issues-in-outlook-for-(pc-ecf61305-f84f-4e13-bb73-95a214ac1230

The Reddit article linked in the first post of this thread (which I did not get to read until now) has some excellent information describing how Microsoft is making wholesale changes to their authentication structure and phasing out legacy methods and even explains why it seems so different for people who use an app password than it does for people who use regular passwords. That being said I cannot see how to force Microsoft Office 2021 Outlook to use an app password. In any event you might be interested in those details at Reddit.

A few days passed, this issue hasn't gotten solved yet. It's extremely annoying.

This is not particularly new as they have been rolling out more secured auth process and forcing it on people since last year. When you add/login to your account, it will do any necessary 2FA/MFA, which is supported by modern auth, through a browser screen. Modern auth is supported by most new email clients, including GMail and MS' own email clients. Once verified, it could not bother you again for a long time or even it sense threat..

What happened here is a fundamental issue on server side. No matter the client, the login or reauth process will fail, even on MS' own Outlook apps.

Anyways, after de-provision my phone on Outlook Web settings and re-add my account on my Android and iOS devices at 2am, it has been going smoothly, for almost 20 hours. I am out of pocket, so I don't know if Outlook desktop app at home is still having the issue.

I suspect a lot of people are/were having the issue and not may know about it. GMail seems to be having some Notification issue with their update from 12/31/2023 as well, some people may not be getting the failure message as a notification.