Exchange Server & External Domain

Hello, I have recently created a domain name under no-ip.org. I have also installed a DC and EXCH 2003 server on the same machine with the domain name same as my no-ip domain but instead of the ending .org I put .local

I am able to send and recieve mail inside my network but not from outside. My question is how can I connect my internal domain with my external domain so I would be able to send mail to external users and get mail from them as well?

Thanks in advance.