Emails not delivering to Comcast.net users

Our customers with Comcast.net email addresses are not delivered. We have verified that exit Mandrill (signed by us).

https://forums.xfinity.com/t5/Email-Web-Browsing/Email-messages-not-being-received-by-comcast-customers/td-p/3200884

It looks like I can PM someone on this thread but I need to post first to enable PM....