Outlook 2016 doesn't accept password or app password

It is very relevant because Microsoft themselves, as I referenced, are the ones suggesting the use of "New Outlook" as a way of avoiding this authentication issue, but there are several of us here going back to page 13 with privacy and security concerns (notably with regards to non-MS e-mail accounts) in regards to how "New Outlook" works (very differently as a proxy) from traditional e-mail clients as per its terms and conditions such that it is not a viable option for us. Hope you understand. Cheers.

@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.

@thegreger,

The only users that will see ads are those using the free version of New Outlook. If you have a paid Office Subscription you will not get ads.

Thunderbird, using OAuth2.0 has not thrown an error message as of late, so perhaps that piece of the fix is working, at least for me, and hopefully others. The email client 9Folders was still throwing authentication errors as of this afternoon.

A quick update from my end - It's been working normally for about 3 hours now. Earlier in the day it's still behaving erratically, randomly prompting for password even when the status showed "connected to MS Exchange"

(Outlook 2013, Windows 10 PC, 2FA + app password)