What is SPF?
SPF — Sender Policy Framework — is a DNS TXT record that lists which mail servers are allowed to send email for your domain. Receiving servers look up your SPF record and check whether the connecting server's IP is authorized. The record ends in an all mechanism that says what to do with everything else: -all (hardfail/reject), ~all (softfail), or ?all (neutral).
example.com TXT "v=spf1 include:_spf.google.com include:spf.protection.outlook.com -all"