Configured Mantis with (fictional) parameters below that matched (I've checked and re-checked!!!) my tester:

$g_phpMailer_method = 2;
$g_smtp_host = 'myhostname';
$g_smtp_username = 'MyUser';
$g_smtp_password = 'MyPassword';

Tester - located in c:\php\includes and this works:

<?php
require("class.phpmailer.php");

$mail = new PHPMailer();

$mail->IsSMTP(); // set mailer to use SMTP
$mail->Host = "myhostname"; // specify main and backup server
$mail->SMTPAuth = true; // turn on SMTP authentication
$mail->Username = "MyUser"; // SMTP username
$mail->Password = "MyPassword"; // SMTP password

$mail->From = "noreply@myhost.com";
$mail->FromName = "Mantis Issue Tracker";
$mail->AddAddress("zzz@yyy.com", "Me");

$mail->WordWrap = 50; // set word wrap to 50 characters

$mail->Subject = "Test phpMailer";
$mail->Body = "This is the HTML message body in bold!";
$mail->AltBody = "This is the body in plain text for non-HTML mail clients";

if(!$mail->Send())
{
echo "Message could not be sent.

";
echo "Mailer Error: " . $mail->ErrorInfo;
exit;
}

echo "Message has been sent";
?>