Yahoo (and AT&T mail) can sometimes block access for third-party apps like Thunderbird unless you've enabled using OAuth2 authentication, or you’ve generated and are using a Yahoo app password.

Steps to Fix the Issue 1. Check Thunderbird Account Settings

Make sure these match the latest required settings for AT&T/Yahoo:

  Go to THunderbird -> Account Settings

   Under your account, choose Server Settings & You wanna enter these values:

  * Incoming server: pop.mail.yahoo.com

  * Port: 995

  * Connection security: SSL/TLS

  * Authentication method: OAuth2

^^^ This is important. Don’t use "Normal password"

  * Username: your FULL email address 

Try that and let me know if that works. If not, we'll try the app password route instead.

Thank you for your response. All of my settings are set to where they should be, with the exception of the Authentication method. OAuth2 is not an option, even though that's what it had been set at previously. The options are Normal password, encrypted password, Kerberos/GSSAPI, and NTLM. Any idea why OAuth2 has disappeared and how to fix this?

hmmm, Let's double check that you are using the correct host/server name. there are two that can work, but one works better with 0Auth. Can you confirm you are using the following currently: pop.mail.yahoo.com

You mentioned in your 1st message 'pop.att.yahoo.com', but Yahoo's current preferred server is pop.mail.yahoo.com (changing this may get OAuth2 to show up on your list).

Let me know if this helped

I was able to solve it by setting up an AT&T secure mail key. Thank you for your help.