SPF record tells the mail receiving server information about validity of sending servers. It specific which mail servers will send out email on behalf of your domain.
To setup or modify SPF, login Control Panel -> DNS -> chooose your domain and click edit Zone.
Here is some sample SPF:
allows domain's MX recordes to send mail for this domain, prohibit all others:
"v=spf1 mx ptr -all"
allows only the specified ips to send mail for the domain, prohibit all others:
"v=spf1 ip4:216.10.244.151 ip4:66.226.27.224 ip4:216.10.240.224 mx ptr -all"
You can find a SPF generate Wizard and details of SPF here:
http://www.openspf.org/