Mail App for MacOS: Signing & Encrypting Email

In the Mail app, you can send messages that are digitally signed and encrypted. Mail automatically discovers appropriate RFC822 case-sensitive email address subject or subject alternative names on digital signing and encryption certificates on attached PIV tokens in compatible smart cards. 

If a configured email account matches an email address on a digital signing or encryption certificate on an attached PIV token, Mail automatically displays the Email Signing button in the New Message toolbar. If Mail has the recipient’s email encryption certificate, or can discover it in the Microsoft Exchange Global Address List (GAL), a lock icon displays in theNew Message toolbar. A lock icon indicates the message will be sent encrypted with the recipient’s public key.