Kelly Martin, editor for SecurityFocus.com, published this fine article explaining how e-mail, in its current form, will never, ever be a secure mean of communication. Even if third-party security add-ons have been developed and piled on top of the current SMTP protocol, we cannot say that these applications have been able to stop the ongoing e-mail war against spammers, viruses, phishers and scammers. I don't think that this war will end anytime soon because the SMTP protocol is too well rooted in current IT infrastructures, and replacing it would probably requires investments of billions of dollars globally.
The main reason we will never win the e-mail war against the spammers-phishers-scammers-botnets and their assorted ilk is we're bound by legal standards that limit the ways we can combat e-mail abuse – unlike in the early days of the Internet. The perpetrators are not bound by the law. source: securityfocus.com