A DMARC policy allows a sender to indicate that his emails are protected by SPF and/or DKIM, and gives instructions on how to proceed if none of these authentication methods are successful.
_dmarc IN TXT "v=DMARC1; p=..."
No specific advice is offered to the receiving MTA.
Advises the receiving MTA to treat any email that fails any DKIM and/or SPF checks as suspicious and perform additional checks or mark the mail as suspected SPAM or whatever local policy is in operation.
Advises the receiving MTA to reject any email that fails any DKIM and/or SPF checks.
_dmarc IN TXT "v=DMARC1; p=reject; rua=mailto:firstname.lastname@example.org"