Preventing e-mail address forgery.

Spammers can forge, or "spoof," a domain's From email address to make their spam look like it came from someone in your domain. To help prevent this, it is recommended that mail sent from a domain to be authenticated.

Authenticating mail from your domain can be done in two ways:

1. by adding a digital signature to your messages that conforms to the "DomainKeys Identified Mail (DKIM)" and 

2. by creating SPF records

DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message that is in transit.  The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for further handling, such as delivery. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication.

More details can be found at http://www.dkim.org

SPF (Sender Policy Framework) is a sender authentication method through which the administrator can specify which hosts are allowed to send email on behalf of a domain. The sending domain adds a specially formatted record into its DNS zone file identifying all authorized mail servers. The receiving mail server checks the sending domain's DNS zone file to see if the IP address from which the message originates matches one of the authorized IP addresses.