I know I've seen this same post recently, but that earlier one seems to have disappeared. Hmm. I'll bring this to the attention of a Comcast mod.
Just FYI, this is an odd problem. Incoming email at Comcast goes through two SMTP servers. In this case the first Comcast server accepts the email from from the outside sender, but when it attempts to send it along to the second Comcast SMTP server it gets rejected. Whoops.