"DomainKeys is an anti-spam software application in development
at Yahoo that uses a form of public key cryptography to
authenticate the sender's domain. dkfilter is an SMTP-proxy
designed for Postfix. It implements DomainKeys message signing and
verification. It comprises two separate filters, an 'outbound'
filter for signing outgoing email on port 587, and an 'inbound'
filter for verifying signatures of incoming email on port 25. This
document is to describe step by step how to install dkfilter for
postfix to deploy domainkeys signing and verification..."