Certificates allow you to communicate with others securely over an encrypted connection, or sign a message confirming your identity to the contact. These settings only apply to S/MIME encryption.
You can import, view, edit (except for your own certificates), and delete your certificates under EditPreferencesCertificates.
If you get the error "Peer's certificate issuer has been marked as not trusted by the user. (-8172) - Cannot add SMIMEEncKeyPrefs attribute" after adding your mail certificate, go to Authorities and enable Trust this CA to identify email users for the certificate.
Your Certificates displays a list of certificates that you own. To add a signing certificate, click Import, select the file to import, then click Open and enter a password.
Contact Certificates displays a list of certificates that you have for contacts. These certificates allow you to decrypt messages as well verify signed messages.
Authorities displays a list of trusted certificate authorities that verify that your own certificate is valid.