Introduction to SPF

SPF stands for “Sender Policy Framework”. It’s an email authentication technique that is used against email spoofing. An SPF record allows a domain owner to publish a list of the domains or IP addresses that should be trusted to send emails for a given domain.

If a domain or IP that is not on the “trusted list” (so not in the SPF record) attempts to send an email from that domain - it will not pass the SPF security check, indicating that it may be a fraudulent or 'spoof' email.

See full article here, including how to implement SPF