We have been being listed at ComCast for spamming without any method to determine why. It appears many users forward their mail to ComCast then use the ComCast interface to view their mail. This in itself seems odd but whatever. 

Then they use their ComCast Mail interface to try to report spam. Guess what you are really reporting as spam? Think about it, if you have forwarded your mail? Thats right you are reporting the server that forwarded the mail to you. Think about it the next time that you are not recieving your mail.

Since ComCast is amoung the list of providers who do not allow whitelisting per user you are blocking the your own mail server for all ComCast subscribers.