Make sure you're using the correct server name for each service (IMAP, POP, SMTP, etc.). Our server names and ports page lists the correct values.
If your email software reports something like "Unknown CA Certificate," this means that you don't have the latest root CA certificates installed in your computer or in your email/browser software. You may be able to fix this by downloading the latest updates to your operating system. Alternatively, you can install the missing root certificate into your client directly. Whether or not this is possible will depend on your client; check its documentation to find out.
The certificate can be downloaded from DigiCert's root certificate page. The required certificate is called "DigiCert Global Root CA."