Just to be a bit picky, how do you know that the machine is sending emails fine if nothing arrives?
Is the return email address correct, and is it one allowed by your smtp server (i presume you are using an smtp server?)
If you use ISP A to access the internet, but send through ISP B's smtp server, things can go wrong. Like wise using A to access and send, but with a return addres that belongs to ISP B