If it all works setting Connection Security to none, then I would expect all to work ok with normal settings and changing as written below.

In Menu app icon > Options (Preferences on Mac) > General Scroll forever to the bottom and click on 'Config Editor' accept the risk In search type: tls.version look for this line : security.tls.version.min Double click on line to open an edit window and enter the number 1 click on OK.

Note: some users report also needing to set security.tls.version.enable-deprecated to TRUE. To create this setting right click in the settings area > New > Boolean. Exit window - top right x

Then you would need to restart Thunderbird to ensure those settings were used.

Already set to 1

Did you create or set if already shown: 'security.tls.version.enable-deprecated' to true and then restart Thunderbird?