TB 78.4 was supposed to automatically migrate Yahoo accounts to OAuth2 authentication, but if it isn't working, you can set the smtp authentication manually:

https://support.mozilla.org/en-US/questions/1309104#answer-1359490

After the authentication is set to OAuth2 for all Yahoo incoming and outgoing servers, remove all Yahoo items in Saved Passwords again, restart TB, and enter the regular account password in the OAuth browser window when prompted.

Hi sfhowes,

many thanks for you reply.

The problem is that the OAuth2 browser windows isn't shown for the IMAP servers when TB is restarted. Javascipt is enabled, I've checked that.

When I try to send an e-mail from one of my Yahoo e-mail accounts the OAuth2 browser window pops up and I can enter the credentials for the SMTP server. But that does not help for the IMAP servers. Even after removing the Yahoo e-mail accounts completely and creating this accounts new there's no OAuth2 browser window showing.

But after installing TB on a different machine from scratch I was able to create the Yahoo e-mail accounts properly with working OAuth2 access to both IMAP and SMTP servers.

I suppose that TB has problems with Yahoo accounts and/or OAuth2 in existing profiles.

Addendum:

I've just ran in an "Programming bug" while trying to create a new Yahoo e-mail account in TB. After restarting TB, the OAuth2 browser window showed for one of my Yahoo e-mail accounts and I could enter the credentials. I was able to access this account once, then it stopped working again and keeps showing the "Couldn't connect to server imap.mail.yahoo.com"

Weird ...

Try disabling you anti virus scanner from scanning secure connections. Ensure Thunderbird is accepting cookies. They are integral to oAuth functionality.

Hi Matt,

no anti virus scanner is installed, cookies are accepted, Javascript is enabled.

And it's working, at least sometimes, because when sending an e-mail from one of my Yahoo accounts the OAuth2 browser window is shown and I can enter my credentials for the SMTP server. Unfortunately it's not working for the IMAP servers.

Hi all,

the issue is finally solved!

I've activated "DNS over TLS" (DoT) in my router recently, this caused the issue with the OAuth2 browser window not showing in TB.

In order to get TB working properly with OAuth2 I had to activate "DNS over HTTP" in TB's network connection settings.

Phew ... that was a long and bumpy ride ... ;-)

I cannot find a setting called "DNS over TLS" (DoT). Is this one of the Connection Security choices? I'm using TBird 60.9.1 Would this option appear in TBird only when the router is set for that method?

I just created a POP account a couple days ago, and after creating an App Specific Password & allowing cookies, using their default Ports, it downloaded the existing mail and I could SEND a test message. But after that one download, downloads fail with the message: > Sending of password for user XXX@yahoo.com did not succeed. Mail server pop.mail.yahoo.com responded: Server error - Please try again later. >

Wonder if pop.mail.yahoo.com server is just down, they are famous for that. According to IsItDownRightNow.com , it is.

Go to to "Server settings" for your Yahoo account. Under "Security settings" choose OAuth2 for authentication. Hopefully when it tries to connect the next time, you will go though a login process and everything is tickety boo henceforth

• https://stackoverflow.com/a/63255601/3553808 : Access WebMail Type Of Email-WebSites From TB-WebBrowser-Tab , or , Configure TB to allow very specific OAuth2 based website-addresses in TB's Cookie-Exception list, etc, etc. (please upvote my Q+A both, inside the linked page, if its useful/helpful).