This is a scary story. I've always figured that instances of the following would happen eventually. While we often take email for granted, the possibilities for abuse are always present. The tale is over on Shakespeare's Sister. Basically, mail with www.AfterDowningStreet.org, an activist site, in the text is being blocked by a particular system. Here's part of the post.

Bright Mail also serves other large ISPs in addition to Comcast.

I asked Brad to email me a message containing the blocked phrase; he did—and I didn’t receive it, although I sent an email he sent immediately after without it. I can only imagine how many emails I have sent or been sent that have not been received. This is the first case I have ever heard of where a spam blocking measure was not based on the email address or IP address of the suspected spammer, or a phrase in the subject line. Perhaps the most outrageous thing about it is that I had no idea that my email was being filtered for this phrase. As far as Comcast and Bright Mail were concerned, infringing on my Constitutional right to organize was so easily discarded that they didn’t even have to let me know.

Other information is available over on The Brad Blog.