SPF stands for Sender Policy Framework and is a technology used to prevent email spam and phishing attempts being sent on behalf of a domain. SPF allows a domain owner to create a policy that specifies which servers are authorized to send emails on behalf of the domain.
This policy is published in the DNS records of the domain and can be used by email servers to check whether a received email actually comes from an authorized server. If the email comes from a server that is not listed in the domain's SPF policy, the recipient may consider it suspicious or spam.
By using SPF, the likelihood of fake emails being sent on behalf of a domain to spread spam or phishing attempts can be reduced. SPF is also one of the technologies used as part of the DMARC (Domain-based Message Authentication, Reporting, and Conformance) initiative to improve email security.