1. How comfortable are you with Exchange ?
2. Which version of Exchange are you using ?
3. Does your server name reflect your domain name ?
4. How many users require external access ?
5. Which features are required for external access ?
If you are anything less than entirely comfortable with Exchange I suggest you call in specialist help and avoid the issues you are going to have to tackle.
Second chances aren't an option when opening up public access to your exchange server.
There are several ways of going about it which can cushion things slightly, but the security implications here are high and many. It's doable, but I don't recommend it unless you're VERY comfortable with Exchange.
In fact, this is the sort of topic I'm reluctant to comment much on since so much can go wrong. Normally I'd insist on either full remote or onsite access to the Exchange server when I set them up, and I'm worried about the potential for a written instruction or suggestion in here being misread, misinterpreted or, well, you get the picture...