Our domain has issues sending emails to comcast.net customers on a somewhat regular basis.

When we send an email to a comcast.net address we get an error indicting too many retries and and it fails to send.  our domain is hosted at DirectNic, cPanel and the techs there have told me they are getting rejected by Comcast servers.  The real unfortunate issue is that it stops sending any email after that for an hour to any address/domain.   this has been an ongoing issue for some time and i see on other posts that others have issues sending to comcast.net customers as well.  can someone help with this issue please!!