Create a new "account" in Thunderbird for the same e-mail address, to test to see if it works better than the old account.

If you are using POP, then first move all your e-mails from the faulty account to Thunderbird's "Local Folders", then remove the faulty account before creating the new account. This will be necessary if you create the new account using POP also.

For IMAP:

1. Thunderbird menu: Tools: Account Settings. (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu)

2. Click the "Account Actions" button on the bottom left and choose, "Add Mail Account".

3. Enter your name, e-mail address, and password, then press the "Continue" button.

4. Continue with the setup until the correct settings are obtained.

5. Thunderbird may probably complain that you already have an account with the same name, and will prompt you to enter a different name for the account you just created.

If the new account works properly, then use it from now on.

Before removing the old account (Thunderbird menu: Tools: Account Settings: Account Actions: Remove Account), make sure that your e-mails are the same, or moved from the old faulty account to the new one.

I have removed and replace with same account and even change the password and it still does not validate this email account. Is there anyway to contact directly support for Thunderbird.

The only support for Thunderbird is through this forum, and we here all unpaid volunteers.

Other people are having problems with AT&T servers today also, so it is most likely a problem with them, no matter what they say.

If you want to test to see if the problem is with Thunderbird or the e-mail server, then Create a new profile in Thunderbird for testing

If it still doesn't work in the new Thunderbird Profile, then the problem is with the AT&T servers.

You should also double-check to make sure you are using the e-mail settings that they say you should be using: https://www.att.com/esupport/article.html#!/dsl-high-speed/KM1010523

Another person using AT&T has confirmed that the problem is with AT&T. See their question here: https://support.mozilla.org/en-US/questions/1156376

I have 2 other emails that is with att and they work without a problem but my one won't work and on the att site to emails it works. So what else can it be.

I get this message: Sending of password for user poppi2005@pacbell.net did not succeed. Mail server inbound.att.net responded

I even removed the program and installed a new one and it did the same thing.

bf2539 said

I even removed the program and installed a new one and it did the same thing.

"a new one", what? A different e-mail program?

no I removed thunderbird and reinstalled a new thunderbird program.

When you uninstall Thunderbird, the Thunderbird Profile folder remains behind. Then when install Thunderbird again, it sees that Profile folder, and automatically uses it.

Thunderbird stores all personal data such as messages, address books and configuration settings in a hard drive folder called the Profile. This folder is separate from the installation of Thunderbird.

That's why I recommend Create a new profile in Thunderbird for testing.

Another person with AT&T problems had success by just re-creating the account in Thunderbird. That didn't work for you, so try the test Profile.