Open Thunderbird's password manager and delete all entries for the affected accounts. Next, open Account Settings > Server Settings for each of the affected accounts and edit the User Name into a full email address. For example, if the affected account's address is usernamepart@domain.com and the username field is currently set to usernamepart, add the @domain.com part to the user name field of that account's settings. Do this for the SMTP server settings for each of the affected accounts and save the changes. Now sync the messages and Thunderbird will prompt for the passwords of the affected accounts and save them afresh.

Well I have discovered in all of the Yehoo/ATT confusion about server names and exotic passwords I somehow managed to get the two accounts I had problems with using the same SMTP server settings. So, when I would go in and straighten out one I would at the same time change the other. So, I think I have them separated and all is well, at least for the moment until Yehoo/ATT makes another change. Thanks for thinking about my problem.

With the aid of Yehoo/ATT OAuth and having 9 email address options I shot myself in the foot. Remember this hoopla related to "Use OAuth or secure mail key for email apps We’re enhancing the security of AT&T Mail. Soon you’ll have to use either OAuth or a secure mail key to get your email through an app."

https://www.att.com/support/article/email-support/KM1240462

I am still not certain what all of that was about. I wasted a lot of time changing to very long and complex passwords. It appeared this was about getting you to change email clients.

add-logic said

It appeared this was about getting you to change email clients.

Not really. OAuth is the industry standard right now, but since not all email clients or apps support it for every mail service provider, you have the choice of using the "secure mail key" as is called by ATT, on those apps. Other providers call it app-specific passwords or app passwords, which you generate for each app that doesn't support that provider's OAuth implementation. For such apps, your regular ATT/Yahoo password won't work, but the generated secure key will work like a normal password. Thunderbird currently supports Yahoo's OAuth, so AT&T's OAuth may also be added in future releases.

Well, the email message was very confusing and had multiple links. It looked like it was written by a nerd software type with a law degree.

I spent a lot of time determining how to get into each of my ATT accounts, drilling down to the right place. At one time doing that with ATT was hell. Fortunately, this time, it was relatively simple and to my amazement I got all 9 umpteen digit passwords and inputted them.

And then had to do one for my wife. But then there was the musical chairs where ATT and Yehoo could not decide where they wanted me to login if I wanted to go to the server. At first it was through Yehoo. Now a login on Yehoo mail goes to an ATT login. And, by the way, my old non-OAth short passwords still work logging into the server.

For those who were not interested in changing clients because they liked Thunderbird it was a somewhat confusing message. I have no idea if some accounts were shut down or not as the threat indicated. I am certain many moved from Thunderbird as a result of the disjoint message. The email message contained this disingenuous message down through multiple links.

"Non-OAuth compatible email apps Outlook 2010, 2013, 2016, 2019 Mozilla Thunderbird Windows Mail on personal computers running Windows 8 or older Apple Mail/Mac Mail on macOS 10.10/Yosemite or older

We suggest you switch to an email app that has OAuth. Get step-by-step help setting up or updating your email account in several popular email apps, with our Troubleshoot & Resolve Tool."

https://www.att.com/support/article/email-support/KM1240462

I hear you.

I have bought a new computer running windows 10. My old computer (now crashed) was running windows 8.1 and Thunderbird version 60-something. Just before the old computer crashed, I had backed it up and stored the profile on an external hard drive. I have successfully restored the backed-up profile to Thunderbird version 68, which is now running on my new computer.

My problem is that I can only receive emails, NOT SEND THEM. I think the reason is that I was in France and had been using a different server on my old computer, so the profile that I have transferred to my new computer has remembered that French configuration. I am now back in the UK and want to simply change the outgoing server details, without losing all my messages. How do I do this please?

Can anyone help?

Many thanks.

Philip.

I am far from being an expert but I will tell you what I do in Thunderbird 68.6.0 (32 bit). I am running Windows 10 64 bit. I open Thunderbird and go to Tools from the top menu and open Account Settings. The left hand window under Account Settings lists all of my email addresses. I select the one I want to modify the outgoing server information for. Then, in the right window at the bottom right is an Edit SMTP server. That opens up your outgoing server settings.

When you make any changes Thunderbird often asks for your password at the point of sending an email. You can find and modify your passwords by going back to Tools and selecting Options. On the left side is Security. That opens other options. Select Passwords at the top. Then open Saved Passwords and then Edit Passwords. When you say Yes to showing your passwords you can see all of them. Right clicking on any one gives you some options. I usually edit whichever I need to change or if I do not need to change a password and just need to satisfy Thunderbird with a password I Copy Password and then close everything down and go back to the Enter Password box and put the password in and check the password manager remember box and see if that works.

As I said I am not an expert and learned the hard way after I did not have a unique user name for every email address I had and had to keep changing user names back and forth. This is the extent of my knowledge.

I hope this helps.