The information in the release notes does mention that there was a change in the TLS so that Thunderbird has set the minimum to use the latest version TLSv1.2 There have been several questions over the past couple of months regarding this issue. It is possible the server you are using may not be using up to date protocols. Suggest you just follow the instructions at link below to reset the minimum version. * https://support.mozilla.org/en-US/kb/thunderbird-78-faq#w_after-upgrading-to-thunderbird-78-i-cannot-get-or-send-email-messages Link above mentions you can use the top right search to locate 'Config Editor', but it is in location - same page at the bottom. How to locate the 'Config Editor' button: * Menu app icon > Options / Preferences * Select 'General' * Scroll to the bottom * Click on 'Config Editor' button