Post your settings, and compare with https://www.att.com/support/article/dsl-high-speed/KM1010523/

Using a secure mail key?

SMTP settings are correct. Have been using ATT secure mail key (i.e. long password)

Using a VPN? What's the antivirus, and its settings for TB?

Your picture is blacked out.

Thanks for responding. I see from past posts that not being able to send is a recurring problem and many of the posts did not receive any response.

  Not using VPN.  Disabled Malwarebytes.  Ran in Safe mode.  Ran TB Troubleshoot option.  No success.  Just now tried sending one email using the "TO" for addresses instead of the "BCC" I normally use and that email appears to have been sent.  At this point it would appear to be some problem with the "BCC" option.  Will need to try some more emails to see if I get the same result.  Any ideas why BCC  appears to have quit working? 
  Tried re-sending the error image but it keeps turning black.  Do not have any idea why,  It is just a small .gif file.

Some providers don't allow messages with just bcc addresses. Put your own address in the To: box and add contacts to the Bcc: box.

Have used BCC for years w/o a problem. Will give your suggestion a try. Tried another email as before - no send w/BCC, sent w/TO. So it appears there is a problem with BCC option. Maybe I should just re-install TB. Thanks again.

Have used BCC for years w/o a problem. Will give your suggestion a try. Tried another email as before - no send w/BCC, sent w/TO. So it appears there is a problem with BCC option. Maybe I should just re-install TB. Thanks again.

About three months ago I started seeing yahoo customers who could not send BCC without a valid To as well. SO I think your problem truly is the provider here.