Go to account settings. The last item in the list of accounts is "Outgoing Server (SMTP)".

Select that and you'll see the SMTP "accounts" that Thunderbird uses. You can select and edit them.

I'm guessing you'll need to set security to TLS/SSL and I'd expect the port number to be 465. If not set already, you probably need to select authentication with password, too.

You really need to go back to your email provider's website and find information on these settings. It's never a good idea to guess.

Your advice is very good. But I find that I also need to change the password for the e-mail connection. I had changed it on the AT&T web site, but Thunderbird must have it stored somewhere else, instead of using the AT&T web site value. Can you please tell me how to change it for Thunderbird? Many thanks for your help!

Tools|Options|Security|Passwords→Saved Passwords

Locate and delete all instances of the defunct password. Restart Thunderbird.

But I'm surprised. I'd expect that with the stored password having failed, Thunderbird would prompt you for the new one.

Wtrebla said

Apparently I need to change something in Account Settings. AT&T sent me an email some time ago, but I saw no way to set things as they recommended. Thunderbird does not have a way to set the port number for outgoing mail, for instance. They said to use port 995 for both, I think. Anyway, on January 10, I suddenly had no access to email on Thunderbird.

To my surprise, the next time I entered Thunderbird, it DID ask me for a new password! I did so, and now it works! Many thanks!