After payment is made using PayPal, the following error message is received from PayPal.
Curl Error! The Curl PHP extension returned the error message: 6 during communication with PayPals IPN server!
The error number 6 has the following meaning: "Couldn't resolve host. The given remote host was not resolved." This indicates that there is something wrong with the DNS on the server, making it unable to find the IP of the paypal.com domain.
HTTPS Proxy Settings.
Some servers are set up with Curl on a proxy server. If the proxy server path is not set, the above error may occur.
See the help topic for Curl SSL.