public static string RenderViewToString(this Controller controller, string viewName, object model)
{
    controller.ViewData.Model = model;
    try
    {
        using (StringWriter sw = new StringWriter())
        {
            ViewEngineResult viewResult = ViewEngines.Engines.FindView(controller.ControllerContext, viewName, null);
            ViewContext viewContext = new ViewContext(controller.ControllerContext, viewResult.View, controller.ViewData, controller.TempData, sw);
            viewResult.View.Render(viewContext, sw);
            viewResult.ViewEngine.ReleaseView(controller.ControllerContext, viewResult.View);

            return sw.ToString();
        }
    }
    catch(Exception ex)
    {
        return ex.ToString();
    }
}

public ActionResult Mail()
{
    // whatever you use to get your User object
    var model = new User();

    var output = this.RenderViewToString("~/Views/User/Email.cshtml", model)
}