Cómo evitar el correo spam () en PHP

<?php
/***
   Email addresses can validate using email_validation(...) method.
   
   This class is use to send mails to valid email address(es)
   by calling methods as following order,
   1) set_header(.....)
   2) set_body(...)
   3) send_mail(.....)
***/
 
class GenerateMails{
   private $headers = array();
   private $recipients;
   private $contenttype;
   private $subject;
   private $email_msg;
   private $hdr;
   
   public $msg;
 
   
   /* This function use for validate email address(es).
      para :
         $email_addrses = This can be list of email addresses seperated by commas
                      or email addresses array
      return :
         true/false
   */
   public function email_validation($email_addrses)
   {
      $validate = true;
      if(!is_array($email_addrses))
      {
         $email_arry = explode(',',$email_addrses);
         $validate=$this->chk_email_arry($email_arry);
      }
      else
      {
         $validate=$this->chk_email_arry($email_addrses);
      }
      return $validate;
   }
   
 
   /* This function use for generate msg for email address(es)
      para :
         $email_arry = Email array
      return :
         true/false
   */
   private function chk_email_arry($email_arry)
   { 
      $rtn_val=true;
      foreach($email_arry as $val)
      {
         if(!filter_var($val,FILTER_VALIDATE_EMAIL))
         {
            $this->msg .= $val.' - Invalid Email Address.<br />';
            $rtn_val=false;  
         }
      }
      return $rtn_val;
   }
 
 
   /* This function use to set sender email & name
      para :
         $email = sender email
         $name = sender name
   */
   private function from($email,$name)
   {
       @ini_set('sendmail_from','test');
       $this->headers[] = 'From: '.$email.' <'.$name.'>';
   }
 
 
   /* This function use to set reply to email & name
      para :
         $email = Reply email
         $name = Reply name
   */
   private function reply_to($rplyto_email,$rplyto_name)
   {
       $this->headers[] = 'Reply-To: '.$rplyto_email.' <'.$rplyto_name.'>';
   }
 
 
   /* This function is use for add reciepients.
      para :
         $recipient_list = recipient list should separate by comma.
   */
    private function add_recipient($recipient_list)
    {  
      $this->recipients =$recipient_list;
    }
   
 
   /* This function is use for add CC.
      para :
         $cc_address_list = cc address list should separate by comma.
   */
    private function set_cc($cc_address_list)
    {
      $this->headers[] = "CC: ".$cc_address_list;
    }
   
 
   /* This function is use for add BCC.
      para :
         $bcc_address_list = BCC address list should separate by comma.
   */
    private function set_bcc($bcc_address_list)
    {
      $this->headers[] = "BCC: ".$bcc_address_list;
    }
   
 
 
   /* This function use to set header values.
      para :
         $header_type = normal - without attachment
                      attachment - with attachment
         $ishtml = true - content type is HTML
                  false - content is plain text
   */
   public function mail_header($header_type, $ishtml = true)
   {
      $this->headers[]  = "MIME-Version: 1.0";
     
      switch($header_type)
      {
         case "normal":
            if ($ishtml) {
               $this->contenttype = "text/html";
            } else {
               $this->contenttype = "text/plain";
            }
             $this->headers[]  = "Content-type: ".$this->contenttype."; charset=UTF-8";
         break;
         
         case "attachment":
            $semi_rand = md5(time());
            $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
           
             $this->headers[]  = "Content-Type: multipart/mixed;\n" . " boundary=\"{$mime_boundary}\"";
         break;
         
      }
   }
   
   
   /* This function use to set priorities
   */
   private function priority()
   {
       $this->headers[]  = "X-Priority: 3";
       $this->headers[]  = "X-Mailer: php";
   }
   
 
   /* This function use to set mail subject.
   */
   private function set_subject($subject)
   {
      $this->subject = $subject;
   }
   
 
   /* This function use to set mail attachmant(s)
      para :
         $attfile = file name(s) array
   */
   private function set_attachment($attfile)
   {
      $semi_rand = md5(time());
      $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
   
      // preparing attachments
      $file = fopen($attfile,"rb");
      $data = fread($file,filesize($attfile));
      fclose($file);
     
      $data = chunk_split(base64_encode($data));
     
      $message = "Content-Type: {\"application/octet-stream\"};\n" . " name=\"$attfile\"\n" .
      "Content-Disposition: attachment;\n" . " filename=\"$attfile\"\n" .
      "Content-Transfer-Encoding: base64\n\n" . $data . "\n\n";
      $message .= "--{$mime_boundary}\n";
 
      return $message;
   }
   
   
   /* This function use to set mail header
      para :
         $header_type = normal - without attachment
                      attachment - with attachment
         $ishtml = true - content type is HTML
                  false - content is plain text
         $from_email = from email address
         $from_name = from name
         $recipient_list = recipient email(s) seperated by commas
         $cc_address_list = CC email(s) seperated by commas
         $bcc_address_list = BCC email(s) seperated by commas
         $subject = email subject
   */
   public function set_header($header_type, $ishtml=true, $from_email, $from_name, $rplyto_email, $rplyto_name,$recipient_list, $cc_address_list, $bcc_address_list, $subject)
   {
      $this->mail_header($header_type, $ishtml=true);
      $this->priority();
      $this->from($from_email,$from_name);
      $this->reply_to($rplyto_email,$rplyto_name);
      $this->add_recipient($recipient_list);
      $this->set_cc($cc_address_list);
      $this->set_bcc($bcc_address_list);
      $this->set_subject($subject);
   }
   
   
   /* This function use to set mail body
      para :
         $email_msg = massege
         $email_msg_type = normal - without attachment
                    attachment - with attachment
         $attfile = file name(s) array
   */
   public function set_body($email_msg,$email_msg_type,$attfile)
   {
     
      switch($email_msg_type)
      {
         case "normal":
            $this->email_msg = wordwrap($email_msg,70);
         break;
         
         case "attachment":
         
            $semi_rand = md5(time());
            $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
     
            $this->email_msg = "This is a multi-part message in MIME format.\n\n" . "--{$mime_boundary}\n" . "Content-Type: text/html; charset=\"iso-8859-1\"\n" . "Content-Transfer-Encoding: 7bit\n\n" . $email_msg . "\n\n";
            $this->email_msg .= "--{$mime_boundary}\n";
           
            if(is_array($attfile))
            {
               for($x=0;$x<count($attfile);$x++)
               {
                  if(file_exists($attfile[$x]))
                  {
                     $this->email_msg .= $this->set_attachment($attfile[$x]);
                  }
                  else
                  {
                     $this->email_msg .= ' No Data found for the file '.$attfile[$x]."--{$mime_boundary}\n";
                  }
               }
            }
/*          else
            {
               $this->email_msg = $this->set_attachment($attfile);
            }
*/
         break;
      }
      $this->email_msg = wordwrap($this->email_msg,70);
   }
   
   
   /* This function use to send mail
      return :
         treu/false
   */
   public function send_mail()
   {
      $this->hdr = implode("\r\n",$this->headers);
      unset($this->headers);
      return mail($this->recipients, $this->subject, $this->email_msg, $this->hdr);
   }
}
 
?>