This is a common issue, especially after the Norton/Thunderbird update. The Norton’s email scanning feature tries to interrupt Thunderbird’s connection to scan for viruses and errors. When Thunderbird connects to your mail server, it often intercepts Norton to scan the mail. Basically, when Thunderbird sees Norton’s “middle certificate,” it doesn’t recognise it as valid and shows an alert like “Thunderbird does not recognise Norton’s secure connection”. * The best way to fix it is by importing Norton’s security certificate into Thunderbird. * You just have to go to Norton > settings > firewall > email protection > turn on the scan incoming & outgoing email option. * Then, open Settings> administrative settings > search certificates > and export the root certificate. * Now, open Thunderbird > settings > privacy & security > certificates > view > authorities. * And, import the certificate, also check the “Trust this CA to identify email users” option. * Restart and check; the warning should have disappeared.