exchange 2003 external pop and smtp

I have a situation coming up that I could use some direction with. I have a small network 50 users with a single Windows 2003 Server Domain. Currently all users have a POP3 email account that runs through our ISP. i am in the process of implementing a brand new Windows 2003 Server with Exchange 2003 on it. The main purpose for this is to share calendars, assign tasks and things like that. I have figured out how to receive our pop accounts through a software called pop grabber this is fine, but when it comes to sending the email how do I configure exchange to send externally, sorry I am a newbie to exchange, Any help or direction would be greatly appreciated. Thanks