Do you have a security program scanning outgoing mail? Perhaps the program updated and suddenly became incompatible with TB. Disable scanning of outgoing (and incoming) mail, if it's in place, and see if you can send.

I turned off my Eset and I still can't send outgoing email, though I still get incoming email.

Thanks for your suggestion.

See this discussion about secure mail keys:

https://support.mozilla.org/en-US/questions/1225036

I read the article and still don't know what to do. I think I need help resetting my Thunderbird settings so that they coincide with att/yahoo.

Thanks for answering my query.

Once you create a secure mail key:

https://www.att.com/esupport/article.html#!/email-support/KM1240308?gsi=4f2qg6

open Tools/Options/Security/Passwords/Saved passwords, click show passwords, right-click the entry for the ATT incoming and outgoing servers, Edit Password, and copy in your secure mail key, OK to close Options. When you restart TB, see if you can receive and send mail.

If you hadn't saved passwords in Options before, you will be prompted to enter the password (mail key) at the next connection, so enter it and check the box to have it stored in saved passwords.

Hi, Thank you for your reply. I'm a bit overwhelmed with the changes I must make at att and thunderbird. I'm not even sure, after doing all of this, that thunderbird and att will sync up. I'm very afraid of losing my incoming messages at thunderbird. That has happened before when I changed passwords. Again, thanks for the help.