That is very unfortunate - I haven't dealt with Comcast a great deal, but I can imagine how frustrating that can be (we've had issues with Hotmail for some of our clients).

I am curious about a few things...

1. Are you sending from Outlook (or something similar), or using an email marketing provider? Email marketing providers will send your message out a handful at a time so you don't bombard mail servers - which gets emails blocked as spam.

2. Are you Sender ID compliant? If Comcast can't validate who you are, it will not trust you.

3. Are you sending on a shared IP address? Just because your messages aren't spam, someone else might be spamming from your IP block. Comcast definitely wouldn't like that.

Your answer might be in the above... or Comcast could easily just suck (like I said, I haven't dealt with them).