Security, more security and even more security!
email2's architecture allows a level of security never encountered before. email2 is not only secure during transit, but it is also secure during storage (it cannot be tampered with), AND recipients are forced to reply via your Private Email Network (PEN), ignoring their email preferences, hardware and software, creating a full audit trail of the entire transaction (conversation), until you decide that it's enough and you recall it.
Public Key Infrastructure (PKI) only protects messages during transit, one way. While they are very robust technologies when it comes time to scramble your message and send them securely across the wire, it still makes use of basic email routes (SMTP) to send your encrypted messages, meaning that it offers NO advantages other than security: it ONLY protects the content of your message ONE WAY.
ALL basic email deficiencies still exist! The security and delivery status of your message is still uncertain. In addition, you are still limited by large file attachments, lack of video messaging, etc.
Since there is no way to enforce that recipients of your encrypted message use the same technique to reply to your message, PKI cannot be considered totally reliable. For non-technical users, PKI may be too complicated to use properly. As the recipients of your message decrypt it, (if they manage to figure out how), they are still able to reply to your message in an non-encrypted way. This defeats the purpose of the initial email encryption!
Finally, PKI requires each user to exchange & manage security certificates with everybody they want to communicate with: ask any email security expert and I guarantee they have many horror stories to tell you about the use of these certificates!
// Edited July 2008