emails to other domains are being blocked

We've contacted customer service serveral times about our emails being blocked when sending to domains other than comcast.

The various tickets are:

                916624283

                916798519

                916626636

As of this date and time, we are unable to email outside of comcast.  One of the agents told us that our emails are being blocked.  The next higher tech was unable to shed light on why.  How can we resolve this bad issue.