The only error I got was "It was not possible to open sendmail input pipe"
and the code I used is like this,as you have provided in the example:
<?php
require('bulkmail/email_message.php');
require('bulkmail/sendmail_message.php');
$message=new sendmail_message_class;
$message->SetBulkMail(true);
$message->SetEncodedHeader('Subject', 'Some subject');
/*
* Define the recipients list
*/
$to=array(
array(
"address"=>"
[email protected]",
"name"=>"abc xys"
),
array(
"address"=>"
[email protected]",
"name"=>"the cellroti"
),
array(
"address"=>"
[email protected]",
"name"=>"saka"
)
);
 /*
  * Create a place holder text message body part
  */
 $text = 'Hello, some text message';
 $message->CreateQuotedPrintableTextPart($text, '',$text_part);
 /*
  * Create a place holder HTML message body part
  */
 $html = 'Hello, some HTML message';
 $message->CreateQuotedPrintableHtmlPart($html, '', $html_part);
 /*
  * Assemble the text and HTML parts as alternatives
  */
 $alternative_parts = array($text_part, $html_part);
 $message->AddAlternativeMultipart($alternative_parts);
 /*
  * Cache the message for all recipients
  */
 $message->cache_body = true;
 /*
  * Iterate for each recipient. 
  */
 foreach($to as $recipient)
 {
   /* Personalize the recipient address. */
   $message->SetEncodedEmailHeader('To',
     $recipient['address'], $recipient['name']);
   /* Send the message checking for eventually acumulated errors */
   $error=$message->Send();
   if(strlen($error))
     break;
 }
?>