SPF is an email authentication standard that helps ISPs better identify legitimate email senders.
Sender Policy Framework (SPF) is an open standard for authenticating email that was originally developed by AOL. SPF attempts to ensure that the IP address from which a message was sent is approved to send email by a domain's owner.
Imagine you own the domain example.com. You also have an email server that uses the IP address 127.0.0.1. You may use an SPF record to designate this IP address, 127.0.0.1, as the only IP allowed to send email using an example.com email address. When an email server receives a message from sender@example.com, it can check the IP address that delivered the message. If the IP is anything other than 127.0.0.1, the SPF check will fail, informing the receiving server that the message is from an unauthorized source.
An SPF record is implemented using a TXT DNS record. The TXT record has several values, the first being v=spf1. This first value indicates that the TXT record is a version 1 SPF record.
The remaining mechanisms in an SPF record are INCLUDE, A, MX, IP4 and IP6, and REDIRECT. These mechanisms specify either an approved IP address directly or a domain that will resolve to an IP address.
SPF is one of several effective ways to check an email's legitimacy, and this article provides only a brief overview. For more information about SPF, see SPF Records Explained.