Are you seeing any error messages? If so, please post screenshots of the error message and of the SMTP server settings pane. thank you

Thanks for your reply. I couldn't get any screenshots, but the error message simply said that it didn't send because it couldn't connect to the outbound server. Or the connection timed out. Ugh!

The settings screen simply says to use system proxy settings. I haven't tinkered with any settings; errors just showed up.

Can I uninstall, and reinstall?

That rarely fixes things and often causes more problems. Try this:

1. click to settings>general
2. scroll to the bottom and, on the right, is 'config editor'. click that
3. enter this: mailnews.tcptimeout
4. I think you will see the default setting is 100. Change to 250 save and exit
5. restart thunderbird and let me know. Also, recheck config editor to be sure your change took.

Well, thanks again for all of your help. So I'm not sure why, but it seems to be working OK again. I ran sfc, and it found and fixed something. Then I rebooted the cpu, and when it came back, thunderbird was working again. My guess was the reboot; that fixes a lot of issues. So for now, it's wait and see, but I'm going to keep your last answer. Just in case. Thanks again.

I was glad to assist. :)