OpenDKIM

OpenDKIM is the reference implementation dealing with DomainKeys Identified Mail (DKIM), maintained by the OpenDKIM Project, licensing under the new BSD License.

DKIM validation aims to prevent email spoofing by checking incoming mail from a remote domain for a DKIM header specifying a signature. The given signature should be validated against a DNS record, held by the remote domain, publishing its public key.
Having your mails properly signed increase your chances to avoid SPAM detection.

DKIM was first defined in RFC 4870, then superseded by RFC 4871, then updated by RFC 5672, and finally by RFC 6376.

Setting OpenDKIM up is a five minutes deal.
The whole setup process is best described by my last puppet module, also dealing with replicating private keys to all your signing SMTPs, and writing your public keys to your DNS and refreshing your zones configuration.