This can't be done using RedirectToAction on it's own. However, the UrlHelper.RouteUrl method can be used to generate absolute URLs using a specific host name.
You simply need to generate the URL using UrlHelper.RouteUrl, then perform the redirect using the Redirect method.
If you know the route name, use something like the following:
var routeValues = new RouteValueDictionary(new { id = 12345 });
string url = Url.RouteUrl("Products.Show", routeValues, "http", "www.domainname1.com");
return Redirect(url);
If you want to generate the URL based on the controller name and action, use the following:
var routeValues = new RouteValueDictionary(
new { controller = "Products", action = "Show", id = 12345 });
string url = Url.RouteUrl(null, routeValues), "http", "www.domainname1.com");
return Redirect(url);
Either of the above would redirect to an absolute URL like http://www.domainname1.com/products/12345
Note that an instance of UrlHelper is available within controller code via the built-in Controller.Url property).