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.

WWW: https://www.libspf2.org/