What is SPF?

SPF (Sender Policy Framework) is a procedure to prevent the forgery of the sender address of an e-mail and to prevent the sending of e-mail via non-legitimate mail servers. It originated as a method to prevent spam.

With SPF, the owner of a domain enters into the Domain Name System (DNS) which IP addresses of mail servers are authorized to send e-mails for this domain.

In addition, SPF is used to determine how e-mail messages sent from mail servers that are not authorized to send messages for the domain are handled (rejection, quarantine or no action).

SPF is not visible to the end user and is checked directly by the receiving mail server.

See also:
Wikipedia: SPF