While I have very little affection or regard for Comcast, my immediate reaction to this would be to pay more attention to the first possibility (Comcast is not *always* wrong), then beef Comcast if you conclude that they are out of whack. Switching to auth'd SMTP would be a Good Thing if you can do it, but it *would* be interesting to know if Comcast's systems did not suck (were correct) in this case.