Hello,
I am the maintainer of a managed package which sends HTTP Posts to an external website configured by the customer using custom settings for the package. The messages provide integration between Salesforce and their external site which hosts scripts we provide.

We have a customer whos website the package is unable to send an httprequest to. It is an https site.

To discover the specific error I attempted a simple post via the developer console. The error is:

System.CalloutException: java.security.cert.CertificateException: No subject alternative DNS name matching customer.website.com found.
(customers website is obviously changed for this example, but I assure you it is the correct URL).

I found a forum post about this type of error here: https://developer.salesforce.com/forums/ForumsMain?id=906F0000000919hIAA

The description said this happens when the sites SSL certificate has its CN (Common Name) set to an IP address, but doesn't have a SAN (Subject Alternative Name) IP Address Record that matches the address in the CN. However after investigating the details of this customers SSL certificate in my browser (chrome), it looks like this customer's CN is NOT an IP address, it is the proper URL ("customer.website.com" in my example). So I'm not sure what the problem is.
I don't know how to tell the customer to fix their certificate, if it's possible.

Our package depends on making an httprequest to a script on an external website in order to achive the integration we provide.
Does anyone know how to prevent this error from happening?

I will paste the full code that I ran anonmously from the developer console when testing. It throws the error mentioned above. I have changed the URL of the customer's site for this example since they probably don't want me posting it in a forum.

HttpRequest req = new HttpRequest(); 
	integer debug=0;
	Map<String,String> headers = new Map<String, String>{};
	headers.put('content-type','application/x-www-form-urlencoded');

	String body='test=1';
	req.setMethod('POST');

	system.debug('content-type is >' +headers.get('content-type'));
	req.setHeader('content-type', headers.get('content-type') );

	req.setBody(body); 
	req.setCompressed(false); // otherwise we hit a limit of 32000
	req.setEndpoint('https://customer.website.com/salesforce/listenerscript.php');

	if ( ! headers.isEmpty() ) { 
		for(string k:headers.keySet() ) { 
			system.debug('setheader ' + k + ' => ' +headers.get(k) ); 
			req.setHeader(k, headers.get(k) ); 
		}
	}

	Http http = new Http();
	HttpResponse res;

	String resultString;
	String resToString;
	String resBody;
	String resStatus;
	Integer resStatusCode=0;
     
	resultString='';
	res = http.send(req);
	resToString=res.toString();
	resBody=res.getBody();
	resStatusCode = res.getStatusCode();
	resStatus = res.getStatus();
	resultString = resBody;
	System.debug(resStatusCode) ;