If you do not get any error from Thunderbird that suggests that Thunderbird doesn't know the email did not go. Couple of things to check: Is Thunderbird in offline mode? Are you definitely connected to the Internet, can connect your email account online? As a test, log in to webmail and send from there. That bypasses all of the communication between Thunderbird and your email provider. Report whether those emails work or fail.

Is Thunderbird in offline mode? Thunderbird is NOT in offline mode.

Are you definitely connected to the Internet...? I know I am connected to the internet because I can browse, and I RECEIVE email messages on my Thunderbird account.

As a test, log in to webmail and send from there. I installed a Webmail account (actually it is an Outlook account). I sent an email message to my wife's account (different computer) and she received it.

What should I try next?

Hi John When I said log in to webmail, you should not have needed to "install" anything. I meant for you to test sending a message from your own account, by opening your own email in a web browser. Can you explain what you mean by 'installed a Webmail account"? Did you set up a new email address, or use the same address you use when sending from Thunderbird?

I obviously did not understand what you wanted me to do. What I did was install a new mail program like Thunderbird - it turned out to be Outlook.

However, I had already tried what I think you are suggesting. I started my browser and opened my Comcast/xfinity program and composed an email message and mailed it to my wife's email address and she received it. So I think that is what you wanted me to do? That was successful.

Note I used the same email address as I use when sending from Thunderbird.

Hi, what you did through Comcast/xfinity is what I meant - and that indicated that it's the connection from Thunderbird to Comcast that is failing, not from Comcast onward. It is strange that Thunderbird does not give an error when sending. Your other test - installing Outlook and setting up your Comcast email to work with it also indicates that Thunderbird has a silent failure. You may know Thunderbird connects to different online servers for incoming and outgoing messages, so if receiving works but sending does not, that's the SMTP server connection, or something else preventing outgoing traffic specifically from Thunderbird. Some standard checks https://support.mozilla.org/en-US/kb/cannot-send-messages 1) can you shut off your virus checker & firewall temporarily, just for a few minutes, and send a few test emails. 2) Start Thunderbird in Troubleshooting mode and again send a few test emails. https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird 3) Since you have it working - check SMTP settings in Outlook, and compare with SMTP settings in Thunderbird. Perhaps change one thing in Thunderbird SMTP settings, close and open Thunderbird, then change that back just to force it to refresh. Again send a few test emails. 4) If you have 'save a copy of sent messages' turned on, are copies of your outgoing messages being saved as expected? Check after doing the foregoing tests and see if some/all are there in Thunderbird. Also check whether those arrive in the Sent folder in Xfinity. More on those suggestions, SMTP settings, https://support.mozilla.org/en-US/kb/cannot-send-messages#w_verify-your-smtp-outgoing-email-settings Please report back after checking those things. If you are thinking about re-installing Thunderbird, be very cautious. With some mystery glitches it might be worth a try. I am especially nervous about suggesting it now that there is a 'profile per installation'. Please first check what version of Thunderbird you have and make sure to install the same version. I hope you have the current release? I would also consider making a copy of your entire Profile before doing that, definitely if you are using POP or have Local Folders. So, last resort only.

I don't know what to say.

I started turning security apps off, changing SMPT settings and sending emails. They all were received! I turned everything back on and emails are still being received.

I am back to my original settings and everything is working fine. Go figure?

I have had this old workhorse computer for 12 years now, and I have had other issues. I am beginning the process of building a new computer to replace this old one.

I will save your final recommendations, and if my email problem returns I will systematically try them.

Thank you so much for your help. I really appreciate your time assisting me.

I feel a little bit like the blind pig that found the acorn!

Thanks again!

So glad that helped! Sometimes just fiddling with the appropriate setting, closing and opening can reset something... Please mark the most helpful post as the solution, for others looking into similar issues.