If sending/receiving works, the issue is in that software so you may need to reconfigure it for Thunderbird.
Do you use any anti-virus or other security software?
As I said in an earlier post, the same happens when in Windows 7 safe mode with network connections.
-Question: where can I see if "Use this proxy server for all protocols" is checked? - Get Add-ons or What's New does NOT load the page.
Connections settings are correct, since an email client other than Thunderbird works with the same settings. Deinstalling and reinstalling Thunderbird doesn't help either. Then I removed all references to Thunderbird, expecting that ZA would ask me for permission the first time Thunderbird was started after that. When I clicked on Firewall - "xxx programs secured" and then there is an 'Add' tab. Read this answer in context Do you use any anti-virus or other security software?
So it has to be a problem with the new Thunderbird update. Selecting that lets you browse to the Thunderbird directory and choose I did not know you could add applications manually that way in ZA. I thought ZA would handle anything not in the list, so I thought removing it from the list would be sufficient. Disable it entirely and reboot to see what happens then.
You could try adding the same provider as a second account in Thunderbird if you have one and see what happens then.
Also, you could enable Logging to see how Thunderbird and the server communicate.
- Do the connections time out silently with no warning/message/logs?
Please let us know what your finding and thoughts are.
If you want a quick solution, it may be best to stick to [https://releases.mozilla.org/pub/thunderbird/releases/52.2.0/win32/en-US/ 52.2.1] for now, or use the restore point.
Possibly this server is unavailable or refuses SMTP connections.
Check of outgoing (SMTP-)server settings are correct and try again.