Comcast Spam Blacklist (RBL) Removal
You send emails to email@example.com and you get a message back from your server that delivery has been delayed. After a couple days you get another message from your server that says delivery has failed. Most spam filters will outright reject your message and tell you why but Comcast simply delays the message forever.
If you happen to have access to your SMTP logs, you can find this information:
xxx.xxx.xxx.xxx Comcast block for spam. Please see http://help.comcast.net/content/faq/BL000000
Reading that page will lead you to this page where you can request to be unblocked.
It would sure be a lot easier if they would reject the message and send this link like everyone else does.