Server doesn't send response code

[Robert Brenstein]

On 28.04.2012 at 22:34 Uhr -0500 J. Landman Gay apparently wrote:
>But PayPal's sandbox test tool consistently shows an error and 
>replies "IPN delivery failed. Unable to connect to the specified 
>URL. Please verify the URL and try again." Obviously it did find my 
>server and URL, because my script received the data and has 
>responded back to Paypal, which in turn has responded with a 
>validation.

No idea how this works in terms of Paypal API but from using the 
online Paypal payments, I know that Paypal normally redirects back to 
seller's webpage, not necessarily the one started with. Is it 
possible that Paypal does not get this URL for post-processing the 
transaction from you and that is what it complains about?

Robert