Reverse DNS Lookup and Sender Policy Framework
Reverse DNS lookup which is another commonly use method in spam filter software, determines the host associated with a given IP address. If an e-mail from externalcompany.com with a source address of 10.10.10.10 arrives at your gateway, the Reverse DNS lookup feature if enabled will check the IP Address is actually associated with where it says it is from, externalcompany.com. If this is not the case, it is assumed the e-mail has been spoofed, and the mail will be classified as spoofed mail or spam.
Sender Policy Framework (SPF)
However reverse DNS is sometimes a problem for some companies. This is because DNS only register an IP address with the DNS name of their spam firewall. A company may send emails from multiple hosts, therefore needs a feature where they can specify multiple IP addresses with this domain name. This is the job of Sender Policy Framework (SPF).
The owner of a domain will publish an SPF record which will consist of authorised senders. When these records are published, the receiver can check the senders records to see if it is associated with that domain, and when the SPF records specify this is the case, the email is accepted. So the SPF record will prove this is a trusted sender for that domain.
Wikipedia's guide to Reverse DNS Lookup