Outlook 2016 doesn't accept password or app password

Yes... When I did that 2 days ago, the error prompt amount and frequency reduced drastically as I posted in my previous comments. I was still getting the sporadic error prompts. In the last 2 days, I gather enough data points the prompts were triggered as a result of me doing something to my emails. I generally have a habit of reading and email then move it to a folder. The move action and delete action is triggering the prompts. I am using the latest GMail for Android app and connecting to eas.outlook.com.

Still having issues, though it seems less frequent. I'm using Gmail for android, using eas (exchange account) to a personal outlook.com address (which doesn't support modern auth, so using app password).

I've totally deleted the account from my phone, cleared storage and cache for Gmail. Deleted the phone from outlook online, cleared all app passwords and generated a new one. I'm now trying to re-add it and getting near instant failure.

Throughout the day it was going down for a few minutes at a time, better than previous days but an issue still exists.

All along my desktop Outlook app (using MAPI over HTTPS) has been working fine, never any issue. Obviously that uses modern auth which does not seem to have had the issue.

The affected Hotmail account in the affected client was deleted and re-added and now SMTP and IMAP are both working as expected in Outlook 2013. I didn't have any issues with other clients though. It just so happened the first client I checked was the only one with an SMTP problem. Thank you for the tip.

You are absolutely correct. I just now made a test using SMTP with Outlook 2013 and immediately and repeatedly get authentication prompts for credentials. IMAP works perfectly, but SMTP seems to fail 100% with a quick test. I should have checked SMTP before assuming resolution based solely on IMAP.

Have you tried deleting all the accounts from the client, then recreating them ?