Try setting up the account without the password. Then, enter the password when prompted at first login. This often resolves that issue.

That allowed me to set up the account, but it would not accept the password on first login. Yahoo mail uses Oauth2 verification and that is not listed as an option. I suspect this is why I can't login. How do I change that setting?

If you look at Server Settings in Account Settings, is OAuth2 a choice in the Authentication method drop-down (also for smtp)? If not, there is a way to set OAuth2 in Config. editor.

Thank you. I did find this setting and changed it to Oauth2. However, it still won't log into yahoo. It sits for a while (5 min) then times out.

I went back and looked and I had left Thunderbird trying to login to yahoo. 5 hours later it was still trying unsuccessfully. So I am still unable to use Thunderbird.

Check that you don't have an antivirus app interfering with the connection, and turn off a VPN if you have one. Check Saved Passwords in Settings/Privacy & Security and remove any obsolete passwords. Also, cookies must be accepted in Privacy & Security. Then restart TB and enter the account password in the OAuth window when prompted. If there are no external apps interfering, the connection should be near instant.

No VPN, Anti virus turned off. I did not find a "Privacy and Security" anywhere.

When I try to send a test e-mail it tells me the password failed and gives me a window to enter a new password. I enter the correct password and get this: "Sending of the message failed. Unable to authenticate to Outgoing server (SMTP) outbound.att.net. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)'." I checked the Outgoing Server settings and Oauth2 is not in the option list.

I found the Privacy and Security settings and cleared the password. Why o why can't programmers put all of this stuff in one place instead of under different tabs located in different areas making it impossible for regular people to find. End of rant.

This problem is on a new install on a new computer. It worked fine (with the same password) on my old computer.

Anyway, I cleared the password and tried again and still get the messages in the last post:

When I try to send a test e-mail it tells me the password failed and gives me a window to enter a new password. I enter the correct password and get this: "Sending of the message failed. Unable to authenticate to Outgoing server (SMTP) outbound.att.net. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)'."

Now that you've stated it's an AT&T account and not a Yahoo account on Yahoo servers (I realize Yahoo provides AT&T mail), I suggest you use the settings given in the link below and create a secure mail key to use in TB in place of the account password:

https://www.att.com/support/article/dsl-high-speed/KM1010523/

Sorry, this has gotten past me. I can't get to my ATT/Yahoo profile, all I get is "Secure Server Failed" messages.

I fail to understand why TB on my old computer worked fine, but will not on my new computer. If I can't resolve this, I guess I'll part ways with Thunderbird.

Secure mail keys were introduced at least a year ago, as a search of this forum will reveal, and most users have made the transition with TB. You can try other mail apps, but it won't change the fact that you need to use a secure key instead of the account password, and create a separate key for each computer.

Unfortunately the instructions you provided do not seem to work. I can not even find "Profile" in my ATT account. It says I am logged in, but nowhere do I find anything to set up a secure key. Sorry, I just isn't working.

When I try to get into ATT, all I get is error messages, so at this time it is not possible for me to set up a secure key.

I think you'll have to contact AT&T directly if the instructions on their own website are incorrect or obsolete.

Yes, I think the root of the problem is AT&T. I keep getting Secure Server error messages. I have had a lot of trouble with ATT e-mail the last few years and If I hadn't had the e-mail address forever I would have switched to Gmail. However I still don't understand why suddenly TB won't work. Thanks for trying.

OK, the ATT server started working again, I got the Secure Key set up and was able to get it into the outgoing server settings, but can not find a way to get it into the incoming settings to get e-mail.

Keep the incoming authentication as 'normal password', delete the entry for the incoming (mailbox://... for POP, imap://... for IMAP) in Saved Passwords, restart TB, enter the secure key when asked for a password at the first connection, check the box to have it stored in Saved Passwords.

The only entry in Saved Passwords is for the outgoing password which is working. When I try to retrieve messages I get a "Host contacted, sending login information". This stays up for a few minutes and then goes away. There is no window to enter a new password.

Check that you have the correct incoming settings: inbound.att.net, 995 (POP server type), SSL/TLS, normal password, User Name = email address.

What is the antivirus? It's not sufficient to 'disable' it - either remove it or exclude the TB profile folder from scanning and disable SSL scanning. Or, run Windows in safe mode to bypass startup apps like AV.

My error. It was still set to Oauth2. It is working now, thanks for your help.