The Sender Policy Framework (SPF) is an email authentication protocol that is designed to help prevent email spam and phishing attacks. It works by allowing email administrators to specify which IP addresses are authorized to send emails on behalf of a particular domain.
When a receiving mail server receives an email, it can check the SPF record for the domain in the email's "From" address to determine if the sending IP address is authorized to send emails for that domain. If the sending IP address is not authorized, the receiving mail server may mark the email as spam or reject it altogether.
SPF records are added to a domain's DNS records and typically include a list of IP addresses and/or ranges that are authorized to send emails for the domain. The SPF protocol has been widely adopted and is now a recommended practice for email authentication.
Done? Test your SPF record.