How do I import a certificate?
The help file is very confusing.
I want to be able to encrypt some of my emails.
Managing S/MIME certificates
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 Edit ▸ Preferences ▸ Certificates.
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.