E-mails not received by comcast.net addresses

Our company e-mails are not being received by employees and customers who have a comcast.net address. We have experimented and "CC"ed e-mails to other addresses, g-mail, yahoo, and our company e-mails. The e-mails are received by other servers but not comcast. We can receive e-mails from the @comcast.net addresses. But even replying to that e-mail is not received by the comcast address. The messages are shown as "sent" and there is no bounce-back message, and the e-mails do not appear in their spam folders. Is there anything we can do about this, or does it have something to do with individuals with @comcast.net addresses?