sorry about the confusion, the recipient needs their own cert, that is why its secure.
how it works: Sender signs and encrypts the message. Only the addressee can decrypt it using their certificate and the sender's public key. If the certificate the addressee has does not match the email address the message was sent to and they don't also have a certificate for the senders address, they can't decrypt it.
comodo has a free certificate offer if you want a certificate so you can see how it works.
http://www.comodo.com/home/email-security/free-email-certificate.php