Small Business Server (SBS) is a line of server operating systems targeted at small businesses by bundling the operating system with a number of other Microsoft products that would normally need to be purchased or licensed separately. The most notable inclusions are Exchange, SQL Server, SharePoint and ISA/TMG (Microsoft's firewall and proxy server).
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You can use a different Exchange server instead, but as I've never done this myself I can't advise you on the detail. I suspect that it won't be quite as simple as it seems...