test.php

<?php
include 'wp-includes/class-phpmailer.php';

$mail = new PHPMailer();
$mail->setFrom('from@example.com', 'From Name');
$mail->addAddress('to@example.com', 'To Name');
$mail->Subject = 'Subject';
$mail->msgHTML( 'Hello ..., ...' );

$status = $mail->send();

echo ($status == true);
?>