I'd remove the old smtp server setting, close Thunderbird, re-open it, add the smtp server over again (taking care to give it a new and unique name), nominate that new setting in the incoming account, then try sending again.

Will that give me two different sets of folders? One for the old emails I imported when setting up Thunderbird, and another for the "new" account?

That is what happened with my father's Thunderbird email, and he keeps getting confused about why there are no new emails in his "Inbox;" when in fact he is looking at the wrong inbox.

I thought I posted this follow-up question last week, and have been waiting for the answer, but have finally figured out how to see the replies and this question was not here.

Thanks for taking the time to help!

No, I am advising that you manually add a new smtp server. That doesn't affect the existing accounts, beyond having to tell the incoming account that you want it to use the new smtp set-up.

Tools|Account Settings will let you go to the account settings page. The last "account" in the list, "Outgoing Server (SMTP)" will let you look at the existing SMTP settings, where you can delete the existing one and add a new instance.

Then you go back up the list to the affected incoming account, select it, and change the "Outgoing Server (SMTP)" box to use the new SMTP server.

Tried that, but one cannot delete an SMTP server unless there is more than one listed. So I created a second one, made it the default (for incoming, too). That didn't help. Then I deleted the first one, closed Thunderbird, opened it again. It still wouldn't ask for a password.

Since my wife was having a similar problem with Microsoft Outlook 2003, I went to our provider's trouble shooting pages for Thunderbird and for Outlook and found one tiny problem with what the provider's tech support person had told me on the phone. The incoming server did require an ENCRYPTED connection, but the outgoing server did NOT (both require authentication, but only the one should be encrypted.) Their help pages also wanted me to set up new accounts, attach them to old accounts, etc., but all I really had to do was un-check the box, under Outgoing server (SMTP), in front of "This server requires an encrypted connection (SSL)."

I was so certain I had set up everything exactly as the tech support person had directed me to, but I guess we need to do our own tech support these days!

Thanks for your help; I wish I could say it solved my problem, but at least I have found a solution. And perhaps my detailing my solution will help other people.