https://core.trac.wordpress.org/ticket/21659

<?php
require 'PHPMailer/PHPMailerAutoload.php';

$mail = new PHPMailer;

$mail->From = 'from@example.com';
$mail->FromName = 'Mailer';
$mail->addAddress('ian.dunn@example.com', 'Josh Adams');  // Add a recipient
$mail->addReplyTo('info@example.com', 'Lukáš');

$mail->Subject = '[test] #21659-core';
$mail->Body    = 'hello from phpmailer directly';

$mail->CharSet = 'UTF-8';
$mail->addCustomHeader( 'Content-Type: text/plain; charset=UTF-8' );
$mail->addCustomHeader( 'Content-Transfer-Encoding: 8bit' );

//var_dump($mail);die('');

if(!$mail->send()) {
    echo 'Message could not be sent.';
    echo 'Mailer Error: ' . $mail->ErrorInfo;
    exit;
}

echo 'Message has been sent';