Outlook 2016 doesn't accept password or app password

No PST file? So all your emails are either through EAS or IMAP and you don't delete emails from the server?

VBA is not supported on the "New Outlook" yet...MS' plan on merging Outlook desktop with New Outlook PWA is not transparent. Either way, "New Outlook" is a no-go for me.

No on the contrary

I delete ALL emails from the servers, no email ever remains there.

Emails not worth keeping just get trashed.

Emails that need retained get exported to the datacenter

Some emails are not emails to be read, they need parsed and the data injected into a database

Others need to have lots of other kinds of actions taken

Things outlook does because it supports VBA and I can literally have it do anything I want

Fortunately no PST-files and I had changed from POP accounts to IMAP some time ago. I always had some problems with those PST-files.

With the latest Thunderbird it was totally easy to create the necessary accounts. I just typesd in the email-address and the password (or even not) and it automatically found the best settings and even connects you to the necessary authorization pages of Gmail, Microsoft etc. I had not a single issue. You just have to install the Addon "TBSync" plus the "Provider for Exchange ActiveSync". I am not a tech-savvy but I set it all up within half an hour.

No PST file? So all your emails are either through EAS or IMAP and you don't delete emails from the server?

VBA is not supported on the "New Outlook" yet...MS' plan on merging Outlook desktop with New Outlook PWA is not transparent. Either way, "New Outlook" is a no-go for me.

I am working on the same... Did you have any large PST file and/or POP3 mailbox that you needed to import to Thunderbird? If so, how was the process? I hate installing or running unvetted process on my main computer and there are a lot of comments about that process doesn't go very smoothly.

Don't get Office/Outlook 2021 yet. First, I have the very latest through M365 subscription on my work computer, and I still have this issue. Second, Office/Outlook 2024 is supposed to come out soon and god knows MS is deprecating things left and right, so spending a lot of money now is not a wise move.

Longer term, you might start look for alternatives. Thunderbird is probably the best free option, but it can be glitchy if you are trying to import large PST files from Outlook initially. You might as well start now.... I don't think the current issue is purely Basic Auth versus OAuth because I should be using OAuth on the latest clients but still having issues. Ironically today, my Outlook 2013 only gave me one prompt, but Outlook for Android with OAuth has given me 114 prompts according to Android notification history. If the 2 MS people are to be believed that they will deprecate Basic Auth soon, then issues like this will only get worse. Give yourself time move things off Outlook.

Also, Proton Mail is coming out with a desktop client, so if you are concerned about privacy, it's another option. It's a decent option to have for those emails that you are willing to keep in the cloud and want to sync across multiple devices. They have calendar, drive, password manager.... so it is a decent one-stop option.

PST files?
I do not use those, those are a disaster just waiting to have a disaster.

I do on the other hand, make heavy use of VBA projects, and if a mail client cannot do those things, it is utterly useless.