I run a small web host and one of our users complained about their mail no longer forwarding last week. I contacted comcast at the address provided in the bounces and got a response within half an hour.

From: abuse-noreply@comcast.net
Date: August 22, 2006 12:10:25 PM EDT
Subject: Comcast.net Blacklist Removal Response

Please do not reply to this message.

This is to notify you that your request for removal from the
comcast.net blocklist has been received.

The following IPs were found within your request. Below each one,
we've included the results of our research.

38.xxx.xxx.xxx

The IP you previously provided has been removed from the
Comcast.net blocklist.

After review of the blocking, the IP you submitted was found to
have been blocked due to the fact that the majority of the traffic
from that IP contained content indicative of spam. If you are not
aware of the traffic that could have caused this, we recommend a
review of your outbound mail logs and ensuring that all computers
connecting to through the submitted IP are clear of any security
exploits.

Thank You
Comcast Network Abuse and Policy Observance