System.CalloutException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 503 Service Unavailable

I'm perfoming a HTTP GET callout to an external production site URL , using Postman I get the response. However, while making callout from Salesforce, it gives the following error:

 System.CalloutException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 503 Service Unavailable

Here is my code:

public static HttpResponse API_Connector(String FullURL) {         HttpRequest request = new HttpRequest();        request.setEndpoint(FullURL);        request.setMethod('GET');        request.setTimeout(60000);        Http http = new Http();         HttpResponse responseObj = http.send(request);        System.debug('Response ----->>>' + responseObj);        return responseObj;    }

Am I missing something else? What could be the issue?

Thanks in advance

#Salesforce Developer