I just read an article by Jim Galvin, published March of 2000 in Information Security Magazine, (IN)SECURITY FROM END TO END.
The article provides an overview of the origination of secure e-mail and how the technologies have changed over the years. It also provides context for digital signatures, e-mail certificates, and PGP versus S/MIME.
Here’s an excerpt from the article:
PGP vs. S/MIME, S/MIME vs. PGP. On the one hand, it really doesn’t matter which of the two technologies you choose. From a user’s perspective, both provide the same set of security services, and neither really has any significant advantage over the other. On the other hand, the fact that there are two choices naturally raises the question of interoperability.