A signed icon (containing a checkmark) is shown in the message header and indicates your message will be signed when you send it.
Address the message to recipients.
An encrypted icon (containing a closed lock) is shown if your keychain contains a personal certificate for every recipient. If you don’t have a certificate for every recipient, click the encrypted icon in your message; an open lock replaces the closed lock, indicating the message will be sent unencrypted.
Some mailing lists reject digitally signed messages because the signature is treated as an attachment. If this happens, click the signed icon in your message; an x replaces the checkmark, indicating the message will be sent unsigned.
Note: If for some reason your certificate isn’t associated with your email address, or if you want to use your certificate with a different email address, Control-click the certificate in Keychain Access, choose New Identity Preference, and provide the requested information.
Open Keychain Access for me
In the Mail app Use personal certificates in Mail on MacUse SSL to connect to the outgoing mail server in Mail on Mac