Thank you Mark, we finally traced the problem down. The VI was configured to use the mail server at his (the developers) location. Like most servers (from what I have read) it has the relay function disabled to prevent spam generators from hijacking the server. Since the e-mail came over the Internet from a remote location, the mail server simply dropped any addresses not located within its own domain.
The fix was to add an input field that allows us to use our own mail sever. Since it is located within our domain, it accepts the e-mail from the application and sends it to all addresses listed without any trouble, thanks again