无法连接到 SMTP 主机

The SMTP server is a Lotus Domino listening on port 25, CentOS machine has NO firewall at all and the strange thing is that even mail() does not work. It returns nothing (while on Windows returns 1). If I send an email through telnet via CentOS server it works just fine so I don't think it is a network problem. It must be related to PHP but I don't know how.

<?php
require("class.phpmailer.php");
$mail = new PHPMailer();
$mail->IsSMTP();
$mail->Host = "192.168.x.x";
$mail->SMTPAuth = false;
$mail->From = "xxx@xxx.it";
$mail->FromName = "XXX";
$mail->AddAddress("xxx@xxx.it");
$mail->IsHTML(true);
$mail->Subject = "Test";
$mail->Body    = "Test";
if(!$mail->Send())
{
   echo "Message could not be sent. <p>";
   echo "Mailer Error: " . $mail->ErrorInfo;
   exit;
}
echo "Message has been sent";
?>

只是为了澄清上面的代码适用于 XAMPP (Windows).

$this->smtp_conn = @fsockopen($host,    // the host of the server
                             $port,    // the port to use
                             $errno,   // error number if any
                             $errstr,  // error message if any
                             $tval);   // give up after ? secs
// verify we connected properly
if(empty($this->smtp_conn)) {
  $this->error = array("error" => "Failed to connect to server",
                       "errno" => $errno,
                       "errstr" => $errstr);
  if($this->do_debug >= 1) {
    echo "SMTP -> ERROR: " . $this->error["error"] . ": $errstr ($errno)" . $this->CRLF . '<br />';
  }
  return false;
}

$mail = new PHPMailer();
// 1 = errors and messages
// 2 = messages only
$mail->SMTPDebug  = 2;