The SPF protocol allows a domain owner to create records in the DNS which indicate definitively which servers are used for sending email for that domain. If an email claims to be sent from within a given domain, but it comes from a mailserver not listed in the SPF data, it can be safely assumed the email is lying about its source, and it can be rejected as spam. When someone makes a spam complaint, the mailservers listed in the header can be compared to the SPF records and it can be determined whether or not the email actually came from that domain.