A SPF (Sender Policy Framework)  is a way to allow you to restrict which mail servers are allowed to send email for your domain name.

An example of an SPF would be : example.com. TXT "v=spf1 -all"

SPF records are TXT records placed into a domains DNS. To setup a SPF record we recommend using the following link:

http://www.openspf.net/dns.html

Please note, forwarding emails can sometimes break the SPF. If this is the case we reccomend using SRS (Sender Rewriting Scheme). More information about this can be found here:

http://www.openspf.net/SRS