Outlook 2016 doesn't accept password or app password

Users that never liked Outlook where using alternative email client even when Outlook 2013 or whatever were a viable option, so?

MS is not forcing anyone to install the new Outlook or buy a subscription, but if you want a free client is there for free.

But if you want to use a MS email account, you need a modern email client that supports OAuth, simple as that.

OAuth has nothing to do with this, you've completely missed the point. The point is that the new Outlook client requires all third party emails to be "cloud cached" onto Microsoft's servers. This data grab is not something Outlook 2013 required, nor indeed any email client Microsoft have ever produced, until now.

Maybe you've only ever used Microsoft accounts with Outlook, in which case this has no effect on you. But for others, sharing all their third-party emails with Microsoft is a pretty big deal.

Please ask the email client provider if they can support EAS with OAuth for Outlook.com. I found there is an EAS add-on for Thunderbird but it must use basic auth because users say it stopped working in late January, Provider for Exchange ActiveSync :: Reviews :: Add-ons for Thunderbird. If you or Rob have not tried this one it might be worth it and to give feedback so the developer will update for EAS with OAuth.

I believe both Thunderbird and the Gmail app connect using the IMAP protocol and authenticate via XOAUTH2.

It's been 4 days and no issues until just minutes ago when the authentication error was received. Outlook.com account on 9Folders on EAS. Thought it was fixed, but alas, issue continue.