SPF is a DNS TXT record on your domain listing which servers are allowed to send email on behalf of your domain. Receiving mail servers check it on every message; without it, they cannot verify whether a sender is authorized, which hurts deliverability and invites spoofing. Publishing SPF is a DNS edit, not a software project.
Also known as
Sender Policy Framework; v=spf1; SPF record
In practice
SPF is verifiable from outside the domain, which makes it a fast first check on deliverability. But SPF alone does not enforce a policy. It only lists allowed senders, and it needs DMARC on top to tell receivers what to do when a message actually fails.
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