Header and development libraries for 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.