How to setup a dmarc record at DNS for Comcast's MS Exchange server?

Our business email with comcast gets flagged as spam more often than I would like.  One of the problems that's been reported is our lack of a dmarc record when a system pings a DNS about us during a SMTP session.  Does anyone here have a clue about how to set a dmarc record up?  I tried it and ended up hosing our email, so I obviosly know enough to be dangerous, but not enough to be successful.

Thanks,

Doug

Below is info about a dmarc record for those not "in the know"

----------------

Error:

Hostname returned a missing or invalid DMARC record

DMARC Records are published via DNS as a text(TXT) record. They will let receiving servers know what they should do with non-aligned email received from your domain.

Domain-based Message Authentication, Reporting, and Conformance (DMARC) is a mechanism for improving mail handling by mail-receiving organizations. The ultimate purpose of DMARC, according toRFC-7489 is to provide a “mechanism by which email operators leverage existing authentication and policy advertisement technologies to enable both message-stream feedback and enforcement of policies against unauthenticated email. Email originating organizations utilize DMARC in order to express domain-level distribution policies/preferences for message validation, disposition, and reporting.

The DMARC Check will return either “Record Found” or “Missing/Invalid DMARC Record”. If the tests returns “Missing/Invalid DMARC Record” then we were not able to find a DMARC TXT record or there were syntax issues with your DMARC Record.

How DMARC Works:

DMARC policies are retrieved by the mail-receiving organization during a SMTP session, via DNS. When mail receivers query DNS, they look for a DMARC TXT record at the DNS domain that matches the one found in the RFC5322. From domain in the email message. If a policy is found, that policy is combined with the author’s domain and the SPF and DKIM results to deliver a DMARC policy result. This policy result will be either “pass” or “fail” and may cause a report to be generated. If a policy is not found, the DMARC module determines the organizational domain and repeats the attempt to retrieve a policy from the DNS.

Having a DMARC message handling policy is currently not required, but recommended as it improves mail-handling between both the sending and receiving organizations.