SSL (Secure Sockets Layer) is an encrypted protocol used to communicate Thunderbird with server. Though Thunderbird, by default, uses SSL while communicating with server.
When mail server tries to communicate with thunderbird via SSL, it send a digital certificate to thunderbird. It tries to check the validity of the certificate. If, somehow, it cannot confirm its legitimate it will display “ Add Security Exception”.
Most of the times this error is taken care itself by mail server. When they realize that error is thrown they change the certificate or replace it. If the problem still exists you need to contact mail server. Or you can create an exception and add the certificate using
“Add Security Exception”.
Go to Tools->Options
- Click the Advanced panel, then click the Certificates tab.
- Click View Certificates, then click the Servers tab.
- Click Add Exception.
- Enter the server name and port as “https://[servername]:[port]”; for example https://test.example.com:995 or https://smtp.test.com:465.
Once it’s been added, you no longer get security exception error