Citation:
If you enter a URL that cannot be found, the IPN simulator will display a message that says "We could not send an IPN due to an HTTP error: 404: Not Found." If you enter an invalid URL, such as a URL that is not proceeded by "Http://" or "Https://", the simulator will display a message that says "Please enter a valid URL." There are a few other cases where the simulator will return and HTTP status code, other than the 200 success code. In those cases, you can look up the message on this page: