Sender Policy Framework (SPF) is an email authentication standard that publishes the list of IP addresses authorised to send mail on behalf of a domain. Mailbox providers reject mail from unlisted IPs.
SPF is published as a single DNS TXT record at the apex of the domain, listing IPs and includes (other domains' SPF policies that should be merged in). It has hard limits — no more than 10 DNS lookups during evaluation — that surprisingly many large senders quietly violate.
SPF alone is fragile: it breaks on email forwarding (the forwarder's IP is not on the original sender's SPF list). It is therefore always paired with DKIM and DMARC for a complete authentication stack.
Missing or misconfigured SPF causes legitimate marketing and transactional email to land in spam, killing conversion rates and customer notifications. It is also a prerequisite for DMARC enforcement.
See it in the product: Email deliverability.