Go to System Services > DNS Server > Zones > Click Records next to the domain.com or your client can do it by SiteWorx > Domains > DNS Records > Click View Records next to the domain.com.
Create a TXT record with the following:
Host: _dmarc.domain.com
TTL: 43200
Text: v=DMARC1; p=quarantine; pct=50; adkim=strict; aspf=relaxed; rua=mailto:dmarc@domain.com; ruf=dmarc@domain.com
You can find more about DMARC here: http://www.dmarc.org/ and you can use https://otalliance.org/resources/spf-dmarc-record-validator to check your DMARC.