This morning Thunderbird greeted me with the following Error:
> [server address:443] uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. > (Error code: sec_error_unknown_issuer)
To trust the certificate (given you know that you want to trust that server) click “Display Certificate” and navigate to the Advanced tab to Export Certificate. You then go to your Thunderbird Settings, Advanced, Security. In the Servers Tab you import the newly exported certificate. Click on it and select Edit Trust to select Trust this server certificate.
Should be sorted! It would really be too easy to place a “Trust certificate” button next to “Cancel” button …