I know there are a number of threads open in regards to SIM and the relay response URL, but since those seem to be resolved and I don't see where they resolved it, I thought I'd try a new thread.

I am having a problem getting the relay response URL to work through the SIM method in the development environment.

I set up my developer account to have a default relay response URL.  I've ensured it's a valid URL and is accessible to the outside world.  It's a simple HTML form just to ensure there are no other issues with response times.  Once the checkout page submits, it the progress bar sits for 5, maybe 10 seconds then it returns the standard error message.  The Transaction Error Notification email  I receive has this in it's text:

Your script timed out while we were trying to post transaction results to it.

   Transaction ID: 0

Transaction Result: This transaction has been approved.

I've tried it with the URL in the x_relay_response_url and without it.  I'm not sure what I'm missing here.

This is one of the last steps I need to get this process rolling (I need to validate the returned form) but until I hit this, I was doing OK.  I had customized the header and got all the form values loaded, so I'm not a complete idiot (though stymiee may disagree on that point from his days at SPF).  But this is baffling me.