Comcast (also known as "Xfinity") is an internet service provider that also provides email addresses to its customers, headquartered in Pennsylvania. Former customers can still have access to their comcast.net address if they've logged in in the past 90 days, but that also means many of these addresses become inactive over time.
smtp;550 5.2.0 Message rejected due to DMARC. Please see http://postmaster.comcast.net/smtp-error-codes.php#DM000001
host xx.comcast.net [x.xx.xx.xx] SMTP error from remote mail server after initial connection: 554 xx.comcast.net comcast x.xx.xx.xx Comcast requires that all mail servers must have a PTR record with a valid Reverse DNS entry. Currently your mail server does not fill that requirement. For more information, refer to: http://postmaster.comcast.net/smtp-error-codes.php#554