SPF Records
Authorizing sending mail servers.
What SPF Solves
SPF (Sender Policy Framework) lets a domain owner publish a list of mail servers authorized to send email on the domain's behalf. Receiving servers check this list to detect forged senders.
SPF answers one question: Is the server that delivered this message allowed to send mail for this domain? If not, the message is suspicious.
Published as a DNS TXT Record
An SPF policy is published as a TXT record in your domain's DNS. There must be exactly one SPF record per domain.
A basic example:
; DNS TXT record for company.com
company.com. IN TXT "v=spf1 ip4:198.51.100.4 include:_spf.google.com -all"