SlackBuildsOrg/libraries/libspf2

libspf2 implements the Sender Policy Framework, a part of the
SPF/SRS protocol pair. libspf2 is a library which allows email
systems such as Sendmail, Postfix, Exim, Zmailer and
MS Exchange to check SPF records and make sure that the email is
authorized by the domain name that it is coming from.
This prevents email forgery, commonly used by spammers, scammers
and email viruses/worms.