Reverse DNS Lookup and Sender Policy Framework
Reverse DNS
Reverse DNS lookup is another commonly used approach in combatting malicious emails and SPAM. Reverse DNS Lookup determines the host associated with a given IP address. If an email from externalcompany.com with a source IP address of 10.10.10.10 is delivered to the anti-spam security gateway where Reverse DNS lookup feature was enabled, the Reverse DNS lookup feature will check the IP Address is actually associated with where it says it is from, in this example, externalcompany.com. If this is not the case, it is assumed the email has been spoofed, and the email will be classified as spoofed email or spam.
Sender Policy Framework (SPF)
Reverse DNS is sometimes a problem for some organisations that send emails from multiple source addresses. DNS only registers a single IP address with the DNS name. Some organisations send emails from multiple hosts, therefore sourcing emails from multiple IP addresses using the same domain name. This can be achieved with the use of a feature called Sender Policy Framework (SPF), which ensures the destination anti-spam solution permits emails from multiple sources using the same domain name.
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 sender's records to see if it is associated with that domain, and when the SPF records specify this is the case, the email is accepted. The SPF record will prove this is a trusted sender for that domain.
Further Reading
Wikipedia's guide to Reverse DNS Lookup