Acronym for Sender Policy Framework
SPF is a way of registering in DNS which machines are allowed to send mail from a domain. This is used to help prevent people from forging emails from people they are not, and hopefully impacting the usefulness of spam.
SPF is implemented in DNS with the use of a TXT? ResourceRecord.
See http://spf.pobox.com/ for more information on SPF
For MTA and distribution specific notes on implementing SPF, see the following:
4 pages link to SPF: