It is now possible to see the TXT records set for a domain, these records typcially contain SPIF records used to help control spam by allowing an email server to verify that the server sending the email is entitled to send it for the apparent sending domain.

Typcially spammers will forge the email "from" address, but they have to use an open or compromised server to actually send the email. Since the SPIF record will not list the used server it can be used to determine that a message is  fraudulent.