Yeah it sounds like it would have to be something with fasthost then. The code we produce doesn't really change at all depending on the email address, so your host must be blocking it somehow.
I would contact them and see if you can access the smtp errors or see if they can explain why the emails are not going through. I don't see how it could be a problem with the code based on your description, so it has to be some kind of setting on the host that they should know more about.
Did this help? Tips are appreciated...
