Why am I not getting some emails?

I have a mailing list for one of my clubs.     My Comcast users are not getting the emails.   Hotmail, gmail, etc. users ARE getting the emails.    I setup a test list with my Comcast email and my gmail email.   I sent a email to the test list.  My GMAIL account gets the email, but m Comcast email does NOT get the email.   It is not in my spam folder or anywhere.

Any suggestions on how to track this issue down?

Dennis