I remember Nirvana had that exact same problem, and had difficulties figuring out why it did not work, but managed in the end. RTM and Toodledo I think never had any such problems. They managed to accept bcc addressed emails straight away.
It seems as if there are at least a couple of different ways to implement email reception, and some of them do not work with bcc - unfortunately I don't know anything at all about this. I hope James and Sergio can solve it.