Melvin Rivera has written a nice tutorial 
for Mail users explaining how to get a digital certificate from Thawte and how to use Mail.app’s digital signature and encryption features.
He outlines the process for creating an account at Thawte and requesting a certificate and then installing it.
Further sections follow on the difference between a digitally signed and an encrypted message, and how to use them.
It’s interesting to compare Melvin’s take on secure email in Mail.app with Matt Haughey’s experience , which wasn’t so positive.
Melvin thinks it works well and is a good tool to have in your email armoury:
Other than the process of going though an external website for obtaining a certificate, Mail’s integration of signed and encrypted messages is seamless. It’s a great feature that is just hidden until needed. Making the user experience simple and clean. And there’s nothing like discovering a great new feature on an App you’ve been using for a long time now.
Joar Winfor has also produced a more detailed walkthrough for secure email in Mail.app, but more detail is not always good for everyone.
