Do you see an error message? What is it?

Yes, sorry for not including the screen shot. Here it is. Thanks so much for your help!

Just to be clear, my iPhone and separate Gmail account send emails to my @twcny.rr.com email address instantly using the same outgoing server settings as Thunderbird and are received immediately by my Thunderbird account.

Try changing to STARTTLS

Thanks for that suggestion. I changed it to STARTTLS as you suggested. Closed and reopened Thunderbird and it never stopped trying to send a test message.. I eventually canceled it and tried again and immediately got an error message ( he same one). Stay with me...

That usually works. Next, try port 465 with SSL/TLS

Update: set to smtp.twc.com with 587 and STARTTLS

Okay, forget that. The challenge is to try different settings. This link offers some suggestions: https://support.mozilla.org/en-US/questions/1408688

You are definitely on the right track. I several combinations and tried the original settings I had (as in the top screenshot), got the same error message. BUT when I pressed OK a window opened asking for my password for the spectrum server. Mind you I am already logged in. And when I put in the password it sent! I sent 3 more tests and they were all sent and received. So I closed Thunderbird, reopened it again, tried to send an email and got the error message as before. However this time when I clicked OK there was no password prompt. Tried many times. I think the server is recognizing a password for every action except sending? Thanks so much.

Would you have any other suggestions as I am still getting same error message. Thanks

Still cannot send. Tried resetting my password, and all suggested combinations of Port and Connection. See attached photos to see where things stand...Current error message, iPhone settings (from my iPhone I can send emails no problems. At present to send emails from desktop Thunderbird, I place them in Draft folder and then send them from my iPhone), Thunderbird settings(same as iPhone)

This may not help, but maybe worth a try: - click setting>general and scroll to bottom to config editor - set mailnews.tcptimeout to a number higher than 100 (which I think is default), possibly 300 - close and exit TB and restart and try again.

Thanks. I did as you said. Indeed it was set to 100 and I changed it to 300. Closed and exited TB. Tried to send but no go. Message is below. Why after 10 years, I think, does this all of a sudden occur. Would deleting my Thunderbird account after backing up my local folders and reinstalling take care of the problem?

Generally, reinstalling does not solve problems. I share your puzzlement, as I see mail.twc.com problems posted here regularly.

When I had timeout issues, it was my antivirus product that was the cause. Relying on Defender only fixed that and saved a subscription.

Thanks for this suggestion. Turned off Malwarebytes and relied on Defender—didn't fix it. I had contacted Malwarebytes a couple of weeks ago and this was their response:

"Malwarebytes has no interaction with Thunderbird, and would only take action if you were trying to download or run something malicious in an email attachment. Malwarebytes for Mac will not block or interfere with something without telling you so in the program or in a pop-up message.

We believe this has something to do with Thunderbird or your Email Service Provider."

Is this a Windows glitch?

I would prefer to see a log of the SMTP failure before guessing. https://wiki.mozilla.org/MailNews:Logging#Thunderbird_Logging_Types

Is this what you are asking for?

I think this is what you wanted to see?...

Wondered if this would be helpful to you to make further recommendations....