...of course, the "good bits" that the commercial version has include the antivirus system, which Mitel isn't at liberty to give away, as they didn't write it.
To answer Andy's question, just about every Linux distribution on the planet will serve http, mail, and ftp. Some, of course, are easier to configure for certain things than others. Andy, what do you want to do that SME doesn't handle? That might make recommendations a bit more useful.