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