Some notes about surface controllers:

• Surface controllers are routed with a hidden field value rather than with a URL. I believe it has a name of "ufprt". I think you can use pretty much any URL so long as that hidden field is passed with the request.
• What makes you think the correct URL would start with /umbraco/surface? On a related note, what version of Umbraco are you using?
• Are you doing a GET request or a POST request? You may want to mark your action method with an AcceptVerbs attribute, specifying both GET and POST: http://www.dotnetexpertguide.com/2012/11/aspnet-mvc-acceptverbs-action-filter.html
• When returning a JSON result from a GET request, you typically have to pass in a second parameter to the Json function to allow GET requests.

Usually looks like this:

return Json(data, JsonRequestBehavior.AllowGet)

That's not necessary if you are doing POST requests.

Thank you Nicholas.

I guessed the url following the documentation

I fixed the controller with the "AllowGet" behavior and now I can get it at /umbraco/surface/ValidationSurface/EmailIsUnique/

Unfortunately the Remote annotation still doesn't work

[Remote("EmailIsUnique", "ValidationSurfaceController", HttpMethod = "GET", ErrorMessage = "UNIQUE")]

It throws an exception when rendering the form field:

Error loading Partial View (file: ~/Views/MacroPartials/Global/Register.cshtml). Exception: System.InvalidOperationException: Impossibile trovare un url per la convalida remota. //Italian exception: means Impossible to find a url for remote convalidation
   in System.Web.Mvc.RemoteAttribute.GetUrl(ControllerContext controllerContext)

I'm trying to get out of this...

public class RoutingHandler : ApplicationEventHandler
    {
        protected override void ApplicationStarting(
            UmbracoApplicationBase umbracoApplication,
            ApplicationContext applicationContext)
        {
            RouteTable.Routes.MapRoute("EmailIsUnique",
                "umbraco/surface/ValidationSurface/EmailIsUnique/",              
                new
                {
                    controller = "ValidationSurfaceController",
                    action = "EmailIsUnique"
                });

        }        
    }

this provide a strange behavior:

without the route /umbraco/surface/ValidationSurface/EmailIsUnique/ calls the "EmailIsUnique" action.

with the route the response is blank, but /umbraco/surface/ValidationSurface/EmailIsUnique/anythingIType calls the "EmailIsUnique" action.

I really don't get why this is happening.

Are you using Umbraco 7? If so, I wouldn't trust that documentation, which may be out of date. As I said, I believe surface controllers are now routed using a hidden field (that gets rendered when you call Html.BeginUmbracoForm). And surface controllers are auto-routed, so you don't need to map a route.

For anybody following this, I explain surface controller routing in a bit more detail in this message: https://our.umbraco.org/forum/umbraco-pro/contour/60788-Umbraco-Forms-and-Ajax#comment-233528

is working on a reply...