If you do not have certificates for everyone to whom the message is addressed, you will be prompted to send the message unencrypted. If you have the public certificate for the user or users to whom the messages is addressed, you will be able to encrypt the email message: In the lower right of the message header, click the open lock icon to lock it when the icon is locked, your email message will be encrypted. You may not want to sign messages to mailing lists, because S/MIME digital signatures are attachments, which some lists do not accept. If the 'Signed' icon has an x instead, your message will not be signed. To send a signed message, verify that the 'Signed' icon has a checkmark ( ✔) in it. If you don't see the 'Signed' icon, select Customize (in the lower left of the message header), and add the 'Lock' and 'Signed' icons. You should see the 'Signed' icon ( ) in the lower right of the message header (this indicates the message will be signed).
Use these instructions to enable Apple Mail to use client certificates to digitally sign and encrypt email. If you want your personal certificate on other computers or devices, you will need to import it on those devices also.
Your certificate is only available on the computer and user account where you install it.
