Problem - Thunderbird Gmail Norton

[king1]

well it is probably no less secure than allowing the security certificate override you have been doing in the past.

The reason this happens is because Norton (and some other AVs) fail to correctly apply their own security certificate which they need to do to be able to analyse the secure email traffic. If you dive into the view certificate area when that SSL error shows up, you will see that it is the Norton/Symantec certificate that is the problem and not the google/gmail one.

Usually the easiest solution is to uninstall Norton - restart the pc, check that gmail works correction with SSL enabled (it will) - then if you're a sucker for punishment, reinstall Norton and it should correctly add certificates wherever they need to go...