Trouble Sending Html Email To Hotmail And Aol Address With Mail() Function

[100yen]

Hi, I run into trouble sending html email to hotmail and aol address using php mail() function. Gmail and Yahoo are fine. But hotmail and aol are failed to receive anything. 
 
Here is my header setting

        $headers = "From: sales@thtc-usa.com\r\n";
        $headers .= "Reply-To: sales@thtc-usa.com\r\n";
        $headers .= "Return-Path:sales@thtc-usa.com\r\n";
        $headers .= "MIME-Version: 1.0\r\n";
        $headers .= "Content-Type: text/html; charset=ISO-8859-1\r\n";

any suggestion please?

[Larry]

I'm really not an expert on this. I've got nothing to add. Sorry!