This Link provides information on items to check when you are unable to send to recipients on the Comcast Network. 


Allso if you will send me a private message with the originating & terminating mail server names along with any available bounce back messages, I can research the issue and engage Comcast Security Assurance if necessary.


For direct assistance with blacklist issues contact Comcast Security Assurance (CSA)@ 1-877-807-6580.