Spammers can sometimes forge the "From" address on mail messages so the spam appears to come from a user in your domain.  To help prevent this sort of abuse, Google is participating in, which gives domain owners more control over what Gmail does with spam emails from their domain. 

Google Apps follows the standard and allows you to decide how Gmail treats unauthenticated emails coming from your domain. Domain owners can publish a policy telling Gmail and other participating email providers how to handle unauthenticated messages sent from their domain. By defining a policy, you can help combat phishing to protect users and your reputation.

Once SPF and DKIM are in place, you can configure DMARC by adding policies to your domain's DNS records in the form of TXT records.  

To learn more about DMARC, and the process involved, please view the following link: Add a DMARC record