If you’re using your own CA (Certificate Authority) to sync your Symbian Nokia (for example E75), you get a message saying the certificate is not trusted. Of course, you can manually just allow it permanently and you’ll be fine, but you might want to add this Root CA certificate to your Symbian?
Download the CA Root certificate as a DER format and .cer file extension. Transfer the file using USB and then use the file explorer in the phone to browse to it. The certificate should be shown, if not you might got the fileformat wrong.
Open the file and choose to Save it. It warns you that the certificate might not be safe – save anyway. It will ask you to put a name (use suggested) and choose what the certificate is used for. I selected all except VPN. It says the certificate has been saved.
Doublecheck if it has been saved in Control Panel -> Settings -> General -> Security -> Certificates -> Root Certificates. Check that your CA is listed. Select it and choose Certificate Info to get some more info.
Now your phone shouldn’t tell you it doesn’t trust the certificate.