SPF (Sender Policy Framework) is a sender authentication protocol. It is a method for a domain to specify which hosts are allowed to send email on behalf of that domain. The sending domain adds a specially formatted record into its DNS zone file identifying all authorized mail servers. A receiving mail server checks the sending domain's DNS zone file to see if the IP address from which the message is originating matches one of the authorized IP addresses. Additional information about SPF can be found at http://www.openspf.org. This web site also includes a wizard which will create SPF records.

Additional information about SPF may be found in the technical specification which is located at the following URL: http://www.rfc-editor.org/rfc/rfc4408.txt.  This specification has been released as an experimental RFC.