this is just to make record straight
First you need a static public IP address and then a registered domain name. Ask you ISP to create an MX record for your domain so that all e-mail addressed to your organization will be routed to your static IP address.
ON your end .. you need a router to do the NAT of your public assigned static IP address to you internal IP's.
On your exchange server go to the Exchange Server Manager and add the internet mail connector if it not activated yet or added.Then configure the settings.
If all this are complied you may recieve and send e-mail in the internet.