Problem mapping HttpHandler --> HTTP Error 404 Not Found

Question

I am having problems trying to map an HttpHandler in the web.config.

This is the relevant config bit:

Answer 1

None of the previous answers worked for me.
I'm using IIS 8.5, .Net v4.0, Integrated, and was still getting a 404 with the following handler config:

I enabled tracing and found the following :

116. -HANDLER_CHANGED 

    OldHandlerName              testEmail 
    NewHandlerName              System.Web.Mvc.MvcHandler 
    NewHandlerModules           ManagedPipelineHandler 
    NewHandlerScriptProcessor
    NewHandlerType              System.Web.Mvc.MvcHandler, System.Web.Mvc, Version=5.2.3.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35 

As you can see it looks like it had correctly picked up the request using my custom HttpHandler testEmail but MVC had stolen it.
I opened my route definitions in RouteConfig.cs and found that adding:

   routes.IgnoreRoute("{resource}.em");

I got it to ignore requests meant for my Handler.
Hope this helps someone - I was tearing my hair out!