Outlook 2016 doesn't accept password or app password

Same password - I do not have or want app password because I am using modern auth across all my devices and have enabled 2FA on my accounts. These are done quite some time ago. It's true that during account add, the modern auth will open a browser session for me to login and perform 2FA/MFA challenge - and not showing the "password prompt." After the setup is completed and when this issue occurs, I am seeing the prompts. Again, it has been improved since the onset of 1/23 as I have noted in my comments. However, it would not call this fixed if many of us are still experiencing this issue, however sporadic.

As far as I can tell, changing of IP address will have high likelihood for triggering this issue. Also, if you initiate any creation or modification of your email, calendar event, or contacts, to which the client needs to sync with the server, this could also lead to the prompt.

Not just the same password, but the same account, tied to your windows login or added to windows. In my case I've never seen any prompt or authentication to add the account to outlook (or at any point after), it just adds, pulling from the account info with the modern auth process that was validated when added to windows. I don't even sign into windows on my personal machines using MS account (I prefer a local account), I just added it to the additional accounts in windows. On work machines the login to the machine is a MS account (using our company's domain and custom modern auth screens) so that takes care of it.

From what I've noticed on my Gmail android, it isn't that sending an email or synching causes the issue, it is that the issue exists and that's what makes it realize it, it can't complete the action, and that's when the "credentials required" comes up. Or it also comes up on its own after a while even if I didn't try to do anything. Bouncing the network usually resolves it.

Today so far, multiple changes of networks and IPs, IPv4, IPv6, all working fine on Gmail.

You are over complicating things. One email address tie to one password, which is one and same account. We are not talking about aliases. The login email address is the account's email address. I also do not sign into the device with this account and computer's login account is a local account, so this is basically an email account being added to the email client. Same for both my work computer and personal computer. My phone is logged into a Google account for the device, so access Play Store and such. Only the email clients, GMail and Outlook for Android, are logged into these effected Microsoft accounts.

My practice is simple, I don't log into a MS account unless I have to, so my Edge browser is not logged into this MS account. I cannot imagine letting MS control even more aspects of my work and personal lives if they can't even work through this login/authentication issue.

You're actually overcomplicating it. You don't have to use the account to log into windows, you don't have to log into edge. You just add the account under "email & accounts" in windows settings. You can then choose to use that account or not for various things. Outlook detects it automatically.

But for work, if you work for a large company, I don't understand how they would allow an offline/local account to authenticate you to company resources. So like I said, the authentication structure is flawed, if you do not want your MS account known to anything except outlook, then yes you're going to have to hack things a bit and your credentials will be lost from time to time after updates or every x days.

MS for better or for worse has moved to the ecosystem model, which is why they've made it so difficult to use an offline account in windows. As far as they're concerned, you should be using your MS account to log into windows which then will authenticate you to things like email, onedrive, optionally edge, and if your company is modern managed, your domain and resources as well. Apple and Google started the trend, MS jumped on board. Same with forced updates.

But then in the same message you say your phone is logged into your google account (which you don't really have a choice, not if you want to actually use most of the features of the phone) - so why no paranoia there? It is possible to use your phone without a google account, but it will be inconvenient and glitchy, exactly what you're seeing with MS.

I am arriving the same conclusion of others and that you are not helping. You are one the asking if it's the same account and not just same password - it's really a silly question to start with. You have no senses why I answered the way I did... Please don't act like you are the only one can setup an email client and others on this thread somehow don't know what to do or don't have good understanding of technology. And no, I am not "hacking around" credentials. This is ALL your wild guesses because you do not have in-depth understanding of how and why people manages things certain way and they are also contradicting your own comments.

Every major platform try to gain access to monetize user data. The only difference is Google doing it with way more transparency than Microsoft. Google also respect user choices and Microsoft does not - this go beyond email/Outlook and go into the OS and Edge. Windows and Edge ignore and revert back user setting choices, especially around data gathering efforts. If I tell Google and Apple to stop, it actually stops, sync those settings across ALL my devices, and don't sneak behind my back and turn things on like Microsoft does, which is why I stay logged in on my phones and not so on my Window's devices. I can easily use a non-Play Store medium to download apps if I want to. This is also why I don't log into a Microsoft device with an account and it is really not your concerns either because it has nothing to do with this email issue. The situation is of Microsoft's email service and its interaction with Microsoft's own email clients. The issues and duration of the problem speaks volume. Outlook can operate regardless if the device is logged into a domain account or not. EAS protocol works regardless if the device is logged into an account or not... Much of your conjectures are nothing but wild guesses and invalid assumptions.