If Comcast's systems decide an email you send violates their terms of service, they just throw it away. There's no bounce message, no notice to you, and no notice to the intended recipient. It's despicable, but that's what they do.
If @ComcastAntiSpam or another employee doesn't help you here, Comcast Security Assurance may be able to tell you what the problem is. You can reach them 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.
The only sure fix is to use a competent, ethical email provider instead of Comcast.