When email is sent to your domain, the sending server checks your domain's MX record. This record tells the world what mail server handles your email. When you subscribe to the SpamHero service, you will change your domain's MX record so that it points to our mail servers instead of your domain's actual mail server. At that point, all your email will be routed to our servers, where your email will then be filtered and just the good mail will be forwarded on to your real mail server.