Are you checking the account "Outgoing servers (SMTP)" for those SSL/TLS settings? or looking at your incoming account settings?

Thanks for the response. I was looking at the incoming account settings. I have now set the SMTP to SSL/TLS, with the result that I no longer have the e-mail immediately rejected. However, now I get either (1) a message that the connection to the SMTP outgoing server was lost in the middle of the transaction or (2) the message that the e-mail is being delivered remains on the screen forever, or at least until I cancel the e-mail.

do you have an anti virus that scans mail? sounds like you might have and it does not cope with encrypted connections. Who is the email provider?

I have Norton antivirus. I turned the e-mail scan off with no change in the ability to send an e-mail. My e-mail provider is frontier.com, which is part of Yahoo.

Apparently the problem was with Frontier Mail, not Thunderbird. Frontier modified its security systems to require SSL for outgoing as well as incoming mail, supposedly on June 10. However it didn't really apply the modifications until June 30. The information that it sent out before doing so covered only a few e-mail programs, not Thunderbird. However, when I searched the Frontier help website I found a video that did apply to Thunderbird. It told me that, as well as modifying my SMTP server to enable SSL, I had to change the outgoing port from 25 to 465. When I did so everything worked as it should, so the problem is now solved.