AT&T are requiring OAuth or a so called Secure Mail Key (which in essence is an app password from what I understand). As TB supports OAuth, please set the SMTP server to OAuth authentication. Your IMAP/POP server for inbound mail is probably already configured for OAuth in TB and therefore works.

Please advise if this suggestion solved your issue.

Thanks for the prompt reply and the assistance. Unfortunately something is still wrong. Here's what I've done and what's happening:

In Tools>Account Settings>Outgoing Server>Edit, I wasn't getting an OAuth2 option, so I created a new identity and deleted the old one. Here's the info for the new identity:

Server name: outbound.att.net Port: 465 Authentication method: OAuth2 Connection Security: SSL/TLS

I tried sending an email and got: "Connection to outbound.att.net failed" If I click the "Enter New Password" option, I don't actually get to enter a password. It just tries sending the message again.

If I hit "Cancel" I get the following: "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 don't understand why it's saying "SMPT" when I have it set to OAuth2.

I went to Tools>Thunderbird Settings>Privacy and Security>Passwords and looked at what is saved as my password. It's a long jumbled mess that I never created.

My reply seems to have disappeared, so I'll try again. Thanks for getting back to me BTW...

I had to create a new identity in Account Settings because it wasn't giving me an option for OAuth2. Here's how it stands now:

Server Name: outbound.att.net Port: 465 Connection Security: SSL/TLS Authentication method: OAuth2

If I try to send a message, I get: "Login to server outbound.att.net failed"

If I click on "enter new password" I don't actually get to do that, it just tries sending the message again.

If I click on "Cancel", I get "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 don't understand why it's saying SMTP, when inbound and outbound are set to OAuth2.

If I go to "Tools>Account Settings>Thunderbird Settings>Passwords>Saved Passwords", and look at the saved login that is there under my email username the password is a long, jumbled mess that I didn't create.

Thanks for your testing and your reply and sorry for the delay in getting back to you.

The error you receive relates to SMTP rather than OAuth2 because the server settings are not working. SMTP is the default protocol for sending email, for authenticating (i.e. get permission to send an email) a password or in modern environments OAuth2 is used.

A "long, garbled mess" for the password entry is a good sign - provided it is against a provider starting with 'oauth:' and containing reference to att.net. It is the authentication token generated for Thunderbird. Just for reference: which protocol (IMAP or POP) and which server name do you use for inbound email? Would be great if you could provide a screenshot.

I really want to get OAuth2 working for you, but as I have no account with AT&T myself I cannot properly test. There seems to be a lot of rather old and likely outdated info on the AT&T support pages, but next to outbound.att.net there are two other SMTP server names provided on the AT&T user forum that you may want to try:

 smtp.mail.att.net
 smtp.mail.yahoo.com

As Yahoo is operating AT&T mail this seems like a valid alternative. Once you've entered the new server names, please close Thunderbird, wait a few seconds to allow any background service to close as well and restart TB.

If these servers do not give you any joy, I would propose to take a step back, skip OAuth2 for SMTP for now and use the secure mail key in order to re-enable you to send emails. Please find instructions on how to generate your secure mail key on the AT&T website. Server for SMTP should be smtp.mail.att.net with port 465, TLS/SSL connection security and 'Normal password' for authentication. This password is you secure mail key and not your account password.

Please advise if you've had any luck with the above...

Incoming server setting screenshot attached...

I'll try the SMTP settings and the other servers and let you know.

I switched to smtp using smtp.mail.att.net I am able to send email. Strange, though that things worked fine before with outbound.att.net. Oh well...

Glad to hear you're back in business. Please mark the case as solved, thx.

Well, it's partially solved. I'd like to get OAuth2 working again for sending email. It would be nice to figure out if it's a TB problem or an At&t problem, but I'll mark it solved