To help reduce spam, domain name owners should configure Sender Policy Framework (SPF). SPF allows email servers (which are configured) to identify messages which are allowed (or not) to use that domain name for sending emails. RFC 4408 defines SPF standards.
SPF helps to reduce spam, for more information please see http://www.openspf.org/Introduction
http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx
http://tools.bevhost.com/spf/
http://en.wikipedia.org/wiki/Sender_Policy_Framework
At minimum the following TXT record should be created in DNS
TYPE: TXT
RECORD Name: <blank>
TEXT: v=spf1 a mx ~all