The SPF record is an entry in the DNS of the mail domain that determines which mail servers are allowed to send emails with senders from this mail domain. The SPF record is evaluated by spam filter systems and can lead to the blocking of a (spam) mail if the sending server is not listed in the SPF record.
How is an SPF record set?

YouTube Video: Prevent Fake Senders with SPF, DKIM, DMARC and S/MIME