You can enable SPF (Sender Policy Framework) in SpamTitan via:

SpamTitan Version 7.04 and later:  System Setup > Mail Authentication > SPF

SpamTitan Version 7.03 and earlier:  System Setup > Mail Relay > Sender Controls

SPF allows the owner of a domain to use a DNS "TXT" record to specify which IP addresses are authorized to transmit e-mail for that domain. When receiving a message from a domain, SpamTitan can check that DNS records to make sure that the mail comes from an IP address that the domain authorizes to send email.

Enabling SPF will only affect mail from sending domains with an hard fail (-all) SPF record. Mail from domains with no SPF record or with an SPF softfail (~all) record will be unaffected.

More information on SPF can be found at:

http://www.openspf.org

https://en.wikipedia.org/wiki/Sender_Policy_Framework