A sender who e-mails a large group of people including me is using my correct address but I do not receive her e-mail until someone else from the group replies to it. It is not in my spam.
If an employee doesn't help you here in the Forums, give Comcast Security Assurance a call at 1-888-565-4329 (from http://constantguard.comcast.net/help/report-abuse/). Note that this is NOT one of the general customer service numbers. The first-line reps there are sometimes not very helpful, so be persistent, and keep at them until you get a satisfactory resolution.
You might ask the sender to contact their ISP as well, as the problem might lie with them.