Comodo offers fully S/MIME compliant Secure Email Certificates FREE for personal use! Free Secure Email Certificates allow you to encrypt and digitally sign email and attachments keeping them secure and confidential:

Encrypt your emails to ensure that the message and attachments may only be read by the intended recipients
Digitally Sign your emails to prove that the message and attachments really came from you and not someone masquerading as you
Digitally Sign your emails to ensure that the message and attachments cannot be tampered with en route via the Internet without the recipient being alerted to the tampering