Sender Policy Framework is a standard by which the owner of a domain uses a specially formed DNS record to advertise which hosts are authorized to send email for the domain.

Sender Policy Framework (SPF) is a technique to prevent e-mail sender address forgery. With SPF system administrators add information about allowed senders for particular domain in a DNS server's TXT and/or SPF record.

Please have a look at the canonical question What are SPF records and how do I configure them.