DMARC is an email-validation system. DMARC was created by PayPal together with Google, Microsoft and Yahoo!. With DMARC an organisation gains insight and control over the email channel. Organisations can use DMARC to protect their brands against abuse in phishing or spoofing attacks.
The DMARC policy you set for your domain is entirely at your discretion.
You can find more information on writing your DMARC policy here: