I am using Magento 1.8.1 version and I want to set the Paypal IPN to update my order status, but when I am running the IPN simulator test from PayPal is does not work throws an HTTP error see this http://puu.sh/hTQaP/af2473710b.png.
Please advise how I can fix this issue.
Finally I found the solution after reviewing in deep. This was cause to due "USER Agent" HTTP Header in request. You can update this on Ipn.php file. Just replace the below line from this function on app/code/core/Mage/Paypal/Model/Ipn.php (Don't apply on core file. Add it on local dir)
protected function _postBack(Zend_Http_Client_Adapter_Interface $httpAdapter)
at line 143 (in magento 1.8.1)
$httpAdapter->write(Zend_Http_Client::POST, $this->_config->getPaypalUrl(), '1.1', array( 'Connection: close', ), $sReq);
$httpAdapter->write(Zend_Http_Client::POST, $this->_config->getPaypalUrl(), '1.1', array( 'Connection: Close', 'User-Agent: your_company_name' ), $sReq);