Setting SPF and DMARC for a domain that is not sending e-mail

Set the SPF TXT record (domain.com) to:

v=spf1 -all

This will inform receiving mail servers that no servers are authorized to send mail from this domain.

Set the DMARC TXT record (_dmarc.domain.com) to:

v=DMARC1; p=reject;

This will instruct receiving mail servers to reject mail that if it is unsigned or when signature verification fails.