Annoying spam feature!
Comcast used to have pretty much all of their client machines in the .clients.comcast.net and .clients2.attbi.net domains, so it was pretty easy to block them; just deny all traffic from those domains and then people coming in through the comcast mail servers can deliver mail to pell without complaint.
Until this week. At the start of this week, I started to get spam (a particularly noxious type of spam driven by a spam zombie that just blatted the mail message out without checking for smtp error codes; it makes my syslog file look really interesting) in german for various casino operations, coming from client-address.or.comcast.net and client-address.wa.comcast.net. Now, Comcast, being a big ISP, doesn't care about spam until their network segments get so crowded that customers start switching elsewhere, but they made up for it by conveniently dropping their client addresses into a special namespace that could be blocked with no pain to real customers. But no more!
A nasty suspicious mind would turn immediately to pink-contract spammers. But not me. I'll just bung .comcast.com into the blacklist and hope that I can set up a whitelist that captures all the traffic coming from the comcast mail servers.