SPF

From Site5Wiki

SPF (Sender Policy Framework)

SPF or Sender Policy Framework allows for a domain name owner to officially recognize or permit an SMTP server as an appropriate or approved source of email for that domain name. SPF is relatively new and is not always recognized by email providers. However, more and more providers are accepting and adopting the technology.

Official Web Site: http://www.openspf.org
See Also: http://forums.site5.com/showthread.php?t=14210

Important Usage Note: If you are using Site5 as your SMTP service provided for your domain names, you must ensure that you have "a:serverhostname.site5.com" implemented within your SPF record. You should alter "serverhostname.site5.com" to reflect your actual servers hostname. Consult with your Site5 SiteAdmin or Reseller welcome letter or contact customer support if you are unsure of your servers hostname.

The following are example working zone entries:
yourdomain.com. IN TXT "v=spf1 a:servername.site5.com ~all"
@ IN TXT "v=spf1 a:servername.site5.com ~all"