modify the method postReply, near line 2007.

$body = $this->addMessageThread($msg['body']);
$email->send($this->getOwner(), $msg['subj'], $body, $attachments, $options);

Thank you diegomesa.

This code works osTicket (v1.9.12):

Add a new method in class.ticket.php.

function addMessageThread($body) {    

	$sql = 'SELECT poster, body FROM '.TICKET_THREAD_TABLE.' WHERE ticket_id=' .db_input($this->getId()) 
.' ORDER BY created DESC';
		
$result = db_query($sql);

$i=0;
while($row = db_fetch_array($result)) {
	if($i > 0) {
	   $body .=  '<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">';
       $body .= $row['body'];
	   $body .=  '</div>';
    }                        
    else {
        $body .= "<br/><br/><h2>THREAD:</h2>";
	}
    $i=$i+1;
}
return $body;
}