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
an implementations of the Vouch By Reference (VBR, RFC5518) proposed standard.

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/