Well email is a complicated beast, so much so that most it's security related features are not included right out of the box. Some of these features are used to help determine spam (i.e. Spammer doesn't usually setup things like SPF or DKIM records). Mail-in-a-box provides a solution to a lot of these other components, thus not requiring the spammer to set much up at all. Making them more reliable to spam engines than before. I'm not saying Mail-in-a-box is a one stop shop spammer solution, but it's pretty close.