By default, the SME Server won't let you host external DNS (without Mitel's ServiceLink product)... which is what you're trying to do.
Most users use a Dynamic DNS Service like easydns.com or dyndns.com, or a few others. This way, if your server gets a new IP from your provider, it can be configured to auto-update the DNS service with the correct new IP of your server.
Then you just use their DNS Servers and create/register as many domains as you like. Direct them all to your server.
Then, on your server you'll need to create a Virtual Domain for each of your domains. And redirect the web content to the appropriate location, either to an IBay, or use ProxyPass to forward requests to an internal server.
Hope that helps.