SPF helps stop spammers from forging mail. SPF works by domains publishing specially formatted "TXT" in DNS records to tell the world what machines send mail for the domain. When receiving a message from a domain, the recipient server can check those records to make sure mail is coming from where it should be coming from.
Pen Publishing systems do not use SPF for message blocking. For customers subscribed to Advanced Email Security Services, SPF record checking is used as an additional criteria for helping determine how "spammy" a message is.
For more information please see http://www.openspf.org/.