Get free digital certificates from CAcert which can be incorporated into your email client.
From the website:
CAcert.org is a community driven, Certificate Authority that issues certificates to the public at large for free. For the enthusiast looking to dip their toe in the water, we have an easy way of obtaining certificates you can use with your email program. You can use these not only to encrypt, but to prove to your friends and family that your email really does come from you.
For administrators looking to protect the services they offer, we provide host and wild card certificates which you can issue almost immediately. Not only can you use these to protect websites, but also POP3, SMTP and IMAP connections, to list but a few. Unlike other certificate authorities, we don't limit the strength of the certificates, or the use of wild card certificates. Everyone should have the right to security and to protect their privacy, not just those looking to run ecommerce sites.