Cannot send email to otehr comcast customers

I have comcast business and I only use one email box.  My  problem is that I cannot  send emails to other comcast customers, residnetial or business.  Whenever I send to other comcast business customers I get a bouince back email stating "

"This is an automatically generated Delivery Status Notification.     

Delivery to the following recipients was aborted after 1 second(s):"

Whenver I send to other Comcast residential customers, including myself, the email never arrives and I  do not get a bounce back email.  It doesn't go into spam either.   This has only happended with other comcast customers, residential or business.   I can send to other email recipents just fine.  I have called customer seervice many times and have had my issue elevated to tier 2 support status but my tickets get closed  without any notification to me.  The problem with a Tier 2 support ticket is that the tech calls but if  I am unavailable I cannot call them back and my ticket gets closed.  This has been going on for about two months  now with no resolution.   One tech told me  that I did not have a blacklisted IP but  I  don't really know one way or the other.   The big probnlem is not being able to get back to Tier 2 support after they call me.  It ends up being a big dead end.  So, I am really thinking about getting rid of the service, which I don't want to do.  Has this type of problem happended to anyone else?