libspf2 is an implementation of the SPF (Sender Policy Framework)
specification as found at:
http://www.ietf.org/internet-drafts/draft-mengwong-spf-00.txt
SPF allows email systems to check SPF DNS records and make sure that
an email is authorized by the administrator of the domain name that
it is coming from. This prevents email forgery, commonly used by
spammers, scammers, and email viruses/worms.

A lot of effort has been put into making it secure by design, and a
great deal of effort has been put into the regression tests.