Glossary · Email
SPF
A DNS record listing which servers are allowed to send mail for a domain.
Sender Policy Framework is published as a TXT record naming the IP addresses and hosts permitted to send on the domain's behalf, ending with a policy for everything else. The receiving server compares the connecting IP against that list.
SPF alone is fragile: it breaks when mail is forwarded, because the forwarding server is not on the original list. That is why it is normally paired with DKIM, which survives forwarding, and both are then tied together by DMARC.
For validation purposes, a domain with a well-formed SPF record is a domain whose owner has configured mail deliberately. Its absence on a domain that claims to send email is a small negative signal.