OpenDKIM is an open source implementation of the DKIM (Domain Keys Identified 
Mail) sender authentication system proposed by the E-mail Signing Technology 
Group (ESTG), now standardized by the IETF (RFC4871). It also includes 
implementations of the Author Domain Signing Practises (ADSP, RFC5617) and 
Vouch By Reference (VBR, RFC5518) proposed standards.

The OpenDKIM package consists of a library that implements the DKIM service
and a milter-based filter application that can plug in to any milter-aware
MTA to provide that service to sufficiently recent sendmail MTAs and other
MTAs that support the milter protocol. 

WWW: http://opendkim.org/