I have my certifcate issued by "TC Trustcenter" in a CER file. Thunderbird 3.0 needs it as a PFX files. So I installed the CER certificate with the Internet options in the Control Panel. Thereafter I exported the certificate including the "private key" and "all certificates in the certification path" to a PFX file.

After importing this certificate with Thunderbird I still cannot decrypt the mail sent to the mail address of the certificate. What else must I do to use the private key? The certificate is valid since I decrypted the mail already on another computer using Outlook. I verified the certificate serial number to ensure that I use the same certificate.