The Sender Policy Framework (SPF) is an email-authentication technique which is used to prevent spammers from sending messages on behalf of your domain. With SPF an organization can publish authorized mail servers that is which is mail servers are authorized to send mails on behalf of your domain. Together with the DMARC related information, this gives the receiver (or receiving systems) information on how trustworthy the origin of an email is. SPF is an email authentication technique that uses DNS (Domain Name Service).
Suppose a spammer forges a hotmail.com address and tries to spam you. They connect from somewhere other than Hotmail. When his message is sent, you see MAIL FROM: firstname.lastname@example.org, but you don’t have to take his word for it. You can ask Hotmail if the IP address comes from their network. (In this example) Hotmail publishes an SPF record. That record tells you (your computer) how to find out if the sending machine is allowed to send mail from Hotmail. If Hotmail says they recognize the sending machine, it passes, and you can assume the sender is who they say they are. If the message fails SPF tests, it’s a forgery. That’s how you can tell it’s probably a spammer.