Outlook 2016 doesn't accept password or app password

It has nothing to do with eas.outlook.com or m.outlook.com, they all resolve to the same thing. But each time you resolve, you get a different IP. Some IPs are working, some are not. Disconnecting and reconnecting your network will have the same effect as changing the server name.

The explanation is that every time you change something, bounce your network, etc you resolve a new DNS IP. Some of their servers are working fine, others are not. So what a lot of people think are fixes/workarounds are in fact just the normal intermittent nature of this issue.

"Overly dramatic"? Ad hominems are not helpful. Four posts in a row is overtly excessive. For your part you seem to have taken it upon yourself to hijack and dominate this thread apparently feeling a need to respond to each and every post, while adding no new information that hasn't already been stated; i.e. repetition. People use all sorts of software and devices, and while you claim to have a configuration that works for you as you have been repeating over and over, it may not be available to others, while the configurations and same authentication technologies that are problematic with MS are not with other major providers using the same clients. If you have something new to add, great, but otherwise feeling a need to reply to every post while repeating yourself is, frankly, annoying when trying to follow the thread and allow other voices to be heard.

I have Outlook 2013 (15.0.5603.1000) on Windows 10, and also configured as Exchange accounts. Got many password prompts today, and the account could never connect successfully all day despite using newly created app password.

Again, Microsoft managed accounts!!! Microsoft has been pushing this on small to mid-size businesses. Our entire company's email is managed by Microsoft through Office 365 (now Microsoft 365) for Business. We are paying them a hefty amount because I see it in my operating budget for the teams that report to me.

I have deleted my email account and re-add the account, and am using the latest version of the client. Just to de-wrinkle one element, I even downloaded Outlook for Android, which is not my favorite, and it still has the same issue. And to put into context, I am a developer with in-depth knowledge, so please don't assume those of us haven't tried things you are suggesting.

I can tell you have very minimal experiences on this because with Microsoft-managed accounts, our in-house IT has very very very limited options in terms of infrastructure level settings or issues - they are ALL controlled by Microsoft.