Usually when the email does not send to a particular address, it will be because the SMTP server is refusing to relay to that person.
This could be because the domain is blacklisted by the SMTP server.
Another possible cause could be that the BCC Address is the same as the From Address. Most SMTP server will not send a message through that is going to the address the message is coming from.
The hosting company can usually look into the error logs for the SMTP server to get an idea on why messages are not going to one address but still sending to the others.