LinuxSecurity points to a CrossNodes article about PKI:
Your company is negotiating a big deal with a partner, making you a bit nervous about the security of exchanging documents via email. There is a non-disclosure agreement in place, but you’d like to be absolutely certain that only the recipients can see the plans for your company’s new product initiative. When the partner emails their agreement to the final version of the proposed deal, you also want to be able to prove absolutely that the email really is from them. Is there a proven technology that can fulfill both needs?