Thats even stranger that you (simon) are being blocked to reply emails. I was under the impression that BTL was not allowing some comcast addresses in, but this implies that BTL addresses are either not being routed out, they are being dropped by comcast or whomever.
Since emails from yahoo, gmail, etc go through, it implies something more like a comcast issue. We had problems with comcast earlier this year in that those users could not get to our website, while everyone else in the world could... turned out to be a router config somewhere within comcast intranet structure.