Outlook 2016 doesn't accept password or app password

One machine is older - but still 4 cores

It was not about the number of core but the processor generation (we are at core 13th generation)

I've been having the same issue..
Fixed for a few days and then came back as noted by others..

I'm using emClient and connecting using Exchange (EWS) and app passwords.

Just wanted to add that if I keep pressing ok on the password prompt after multiple attempts I eventually get connected and then works ok for hours.. Unless i flush dns, or restart my network adapter which then asks for password again.

Funny thing is that on emClient forums they insisted and still do, that this is happening because Microsoft had announced that they plan to disable all basic authentication back in 2022 and now this change is taking effect for all servers.
Microsoft and basic authentication - personal outlook accounts - Announcement - eM Client
Is this in any way valid? No matter how many times I tried expressing my doubt on this they kept replying the same thing..
I even sent them this post by Microsoft which points to this thread while it was on INVESTIGATING status..
Eventually they suspended my account and locked that thread.. :D

In any case my guess is that if this was the case it would not work no matter how many times I tried logging in..
And also as I already said it worked just fine for two days as noted by others in this thread..

Core i14 is out now. My core 2 (not i2) is basically generation 0. No problems with it, outlook has been fine.

According to MS this is not related to any planned elimination of authentication protocols, etc. If you are using app passwords you should be all set, but most are still seeing the problem. When you reset your network or DNS, you get connected to a new server, and it seems some servers are having the problems and some aren't. When the problem happens for me, I have to bounce the network once or twice and it will go away as I get a new server that has the fixes applied.

The best way to go is modern authentication, but not all apps support that, at least not for outlook.com addresses.

Bounce your network connection (let it stay down for 30 seconds) and see if you can reconnect, may have to try it a couple times.

The reason it works if you keep hitting OK is eventually it tries another server (as you've likely gotten temporarily blocked from the server you were retrying to repeatedly, a security feature) and happens to get one that works that next time.