One of the major changes in update to 78 was the minimum TLS level that is set as default. https://www.thunderbird.net/en-US/thunderbird/78.0/releasenotes/ If your server is not using the up to date TLS protocols then this will stop connection to and from server. Please try the information at the following link to reset the minimum to same as used in versions prior to 78 and then restart Thunderbird. https://support.mozilla.org/en-US/kb/thunderbird-78-faq#w_after-upgrading-to-thunderbird-78-i-cannot-receive-or-send-email-messages Image as guide to find 'Config Editor' using 'search' in Options Alternative - do not use search - select 'General' in left pane and scroll all the way down to the bottom.

Оk. Thank you very much. Now It works correctly.

Good to hear all is now ok. Could you please select my previous comment as a 'Chosen solution' so this helps others when searching for help.