Outlook 2016 doesn't accept password or app password

It is not the IP address assigned on your end (source IP). It is the destination IP returned by DNS when your client looks up "outlook.com" or whatever you've specified for server (there are many aliases that all point to the same place). They load balance amongst many, many servers and only certain ones seem to have the issue.

My guess is if you are dormant for a while the connection goes away (or maybe a blip on your internet, weak wifi signal, etc) then when it re-establishes it hits a different IP at MS. Bouncing your network connection will get you a new random server (that hopefully doesn't have the issue).

@Robdstef,

If you have M365 Personal the best experience would be to use the M365 Outlook that is part of your subscription. It connects with modern auth and is not affected by this recent issue. If you prefer Thunderbird, it would be best to follow the guidance of Thunderbird to use OAuth as noted in this article, Microsoft OAuth Authentication and Thunderbird in 2024 | Thunderbird Help (mozilla.org). I will keep sharing feedback to the service team to work on addressing the issues with basic auth, but you could skip all that with the other two options you have. Modern auth is also a more secure connection method so worth the effort to switch if you can.

This is, to the best of my knowledge, not true? I have this issue, and I'm running version 2401 (Build 17231.20194 CtR) with M365. The issue doesn't seem to be isolated to older builds.

For god's sake, must you keep torturing your PAYING users with this nightmare? Microsoft, just roll the **** back already.

Absolutely it's not true. Nearly 50 pages of comments -- new ones daily -- imploring Microsoft to do the right thing and end this hell by rolling back the changes made nearly a month ago.

Another side effect of this off again/on again issue. Email settings being reset in client to default, requiring re-establishing said settings frequently.