Try to change in THUNDERBIRD preferences editor security.tls.version.min from 3 to 1

From the menu at the top right, go to Options. Scroll all the way to the bottom and click on Config Editor. Skip past the warning. Scroll down until you find security.tls.version.min (or paste security.tls.version.min to upper frame) Double click on it, and set the value to 1

Mozilla disabled TLS 1.0 and 1.1 in the latest versions of Firefox and Thunderbird. Thunderbird 73b and above. This causes issues with anyone connecting via SSL/TLS/StartTLS to services uses 1.1.