Outlook 2016 doesn't accept password or app password

It makes sense. My failure rate on T-Mobile network was nearly 100%. I have T-Mo for phone and home internet. At my work with Comcast, it was much lower. I suspect the load balancing probably has region and internet backbone related decision factors.

Here are my observations from a networking perspective.

I suspect the intermittent behavior is due to the fact that there are many servers associated with eas.outlook.com, outlook.office365.com, and m.outlook.com (which are all basically the same thing). My guess is some servers are having issues, others aren't. Your phone/computer/etc will cache a DNS entry for a certain amount of time, then look it up again, at which point it gets a non-working server and you're down until that timeout expires and it tries again (and happens to get a working one)

Basically it appears the DNS hierarchy is

Main hostnames (which are all just aliases/cnames):
eas.outlook.com

outlook.office365.com

m.outlook.com

outlook.ms-acdc.office.com

www.outlook.com

Looking up any of these will return one of two servers (based on the timing and DNS server):

ooc-g2.tm-4.office.com (strangely sometimes this comes back as a cname/alias for the one below, other times it comes back as its own A record)

MNZ-efz.ms-acdc.office.com

Each of those two servers above has many IPs associated, for load balancing reasons. Doing multiple NSlookups over time or to different servers will return a different list of IPs each time.

What I can't tell without being at Microsoft is if those IPs are themselves load balancers with many servers sitting behind them (which may make any workaround impossible, unless those load balancers are doing SSL offloading and that is where the issue lies, in which case avoiding the problematic ones could still fix it).

For example right now I'm connected to 52.96.58.98 and that is working fine. Earlier I was connected to 52.96.35.178 and that was also working fine. 52.96.19.82 seemed to be fine but it changed before I could test it thoroughly. I just bounced my wifi and am now on 52.96.109.146 and that is working also.

One possible workaround

Put one of the working IPs in for server name. This will only work if you have the "accept all certificates" option, like I do on android. Otherwise it will have a certificate failure. There are some security concerns with doing this though.

The other is to edit your hosts file to point eas.outlook.com or whatever of the main servers above you're using to one of the working IPs, so it won't use DNS and will stick to that IP.

I haven't tested either one as currently I'm working fine, but if it goes down again I'll give it a shot. Again, if the IPs are just front end load balancers, it probably won't help,

It also won't help if the issue is on some authentication server sitting on the backend somewhere, which you can't control.

EDIT - Just go bounced to 52.96.109.226 and that seems to not be working. Switching to mobile data fixed it. Back on wifi and a new IP, and it is working again.

I can confirm putting an IP in the hostname and setting it to "accept all certificates" does work. I guess if it goes hard down again at some point I'll give that a try and see if it actually fixes it.

Edit jan 30 - sadly the workaround does not work, servers that were fine last night are giving the error now, oh well.

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.

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

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.