joebowen
Have you read the manual, I'm sure that is answered in there.
Look on the smeserver.org link at top of forums.
Basically you would do:
Create a domain name & have someone host the external DNS records for you, eg whoever you purchased your domain name from or a free one at DynDNS etc.
Get a ADSL service with a ISP either a static or dynamic IP.
Make sure you select a decent quality ISP plan that allows traffic on ports 80 & 25 and doesn't block ports generally speaking.
If a static IP then configure your DNS records to point the domain at your servers static IP.
If a dynamic IP then you need to use a Dynamic DNS management service such as DynDNS or Yi etc, and configure that service in the sme server setup screens.
Configure your server with your main domain name
Add users to your sme server.
The sme server smtp mail server will now receive mail addressed to user@yourdomain.com
Configure your Windows workstations to collect mail from your server eg mail.mydomain.com & also to send mail via your server.