SPF stands for Sender Policy Framework that facilitates a validation process to allow email receivers to find out if the Simple Mail Transfer Protocol(SMTP) relay is allowed to relay email for users from a site domain like aol.com, yahoo.com, gmail.com etc.

This helps mark email messages as SPAM sent using someone else email address by the email receiving also known as Mail Exchanger(MX) servers. Enterprise email systems often use SPF framework to prevent SPAM messages delivery to their employees.

It is one of the industry leading standards to identify SPAM. The alternate frameworks to identify SPAM are Domain Keys IM aka DKIM and Sender ID.

SPF implementation depends on the internet’s lifeline Domain Name System TXT Resource Record for a site domain in the authoritative DNS server. Presently, SPF version 1 specifications are in use that helps site domain owner to publish the authorized SMTP relays.

SPF mechanism prefix are “+” Pass, “-“ Fail, “~” SoftFail, and “?” Neutral. Evaluations of an Domain SPF Records and Intended Actions are given below:
Evaluations of an Domain SPF Records and Intended Actions
It is highly recommended that you setup SPF or simmilar framework like Sender ID or DKIM for your mail domain to allow MX servers to identify email SPAM and to ensure outbound email delivery. It also helps you protect your brand image by not allowing the SPAMMERS to use your mail domain to send emails.