Proving and Protecting Email Sender Identity
“Email spoofing - the forging of another person's or company's email address to get users to trust and open a message - is one of the biggest challenges facing both the Internet community and anti-spam technologists today. Without sender authentication, verification, and traceability, email providers can never know for certain if a message is legitimate or forged and will therefore have to continually make educated guesses on behalf of their users on what to deliver, what to block, and what to quarantine, in the pursuit of the best possible user experience.“
If you manage mail servers you should follow DomainKey technology.