It may be that the second account is attempting to use the SMTP server of the first account. you can check that by - click 'account settings' - highlight the account that doesn't work in left sidebar - now, in lower right side of screen, click the button for 'edit outgoing server' and see if it is the same email address as the account. if not, revert to prior screen and select the dropdown menu of outgoing server and select the appropriate SMTP server.

Thank you for your suggestion. Although I've always used the same outgoing server address for both accounts, I tried different options as you suggested, but got the same response. :-(

Puzzling. Possibly, if you post screenshots of the SMTP server settings for the two accounts and incoming server settings for the two accounts, someone here may have suggestions.

The SMTP server settings for the two accounts and incoming server settings for the two accounts are they same as they were in 2011! There is no answer in the settings. There is no CHANGE in the settings.

I think the difference is that I am indicating there is a problem in the setup and you are convinced that nothing is wrong. If nothing is wrong, then we need do nothing. If something is wrong, then an investigation seems prudent. Since you state that nothing has changed, your solution should be to your email provider.

As I noted earlier, I have compared the settings between the two accounts. I have compared the settings to those on my other computer. The other computer works fine. I have sent from it without problems since this issue arose. Ergo, it is NOT the email provider, and it is NOT in the setup! I am now convinced that one of the files in this T-Bird program has somehow been corrupted. The issue is to find which file and replace it without compromising many years of saved info by losing my total profile for this account.