What's the antivirus? This issue often occurs if the AV is scanning outgoing mail or secure connections. Try adding the TB profile folder as an exception or exclusion in AV settings, and disable scanning of secure connections.

Thank you for this input. I use Avast Premium and Malwarebytes Premium (with web protection turned off). I made the suggested changed and it seems to work. So I will call it solved with a big thank you!

Hi, I originally marked the response above as solved. I use Avast Premium and put thunderbird as one of the allowed apps. I put the Thunderbird exe file on allowed list previously. I was getting either outgoing server timing out or connection got was lost while sending.

So I looked up every .exe file related to Thunderbird and put it on the allowed list. I then put the profile on the exception list. Current status is that I am getting this message when I send mail

"Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors. You can retry or save the message locally to Local Folders/Sent-larry.cohen0829@comcast.net (IMAP)".

Anyone have some ideas?

Thanks, Larry

First, check your smtp settings: smtp.comcast.net on 587, STARTTLS, or 465, SSL/TLS, User Name = email address (or possibly just the part before @), authentication = normal password.

https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

https://support.mozilla.org/en-US/questions/1320016#answer-1380031

See also https://support.mozilla.org/en-US/questions/1299140