使用带有PHP表单的expressmail

We have a php script that emails form field values when a user submits the form. The form action points to the script below.

We've been asked to configure things to use expressmail explicitly. My question is, would this entail a modification to the script or is this a config setting on the server somewhere?

<?php

if (! $_POST) {
    header('HTTP/1.0 405 Method Not Allowed');
    exit;
}

$redirectTo = html_entity_decode($_POST['post']);
$body       = '<html><body>';
$content    = array();

    foreach ($_POST as $key => $value) {
        if ('-label' !== substr($key, -6)) {
            continue;
        }

        $field = substr($key, 0, strlen($key) - 6);
        $content[$field]['value'] = $_POST[$field];
        $content[$field]['label'] = $_POST[$key];
    }

$body .= '<h1>' . htmlentities($_POST['formName'])  . '</h1>';

foreach ($content as $field => $value) {
    $data  = $value['value'];
    $label = $value['label'];
    $body .= '<p><b>' . htmlentities($label) . '</b><br />';

    if (false === is_array($data) && (null === $data OR "" === trim($data))) {
        $body .= 'N/A';
    } elseif (is_array($data)) {
        $body .= '<ul>';
       foreach ($data as $val) {

            $val   = htmlentities($val);
            $body .= '<li>' . $val . '</li>';
        }

        $body .= '</ul>';
    } else {
        $body .= htmlentities($data);
    }

    $body .= '</p>';
}

$body    .= '</body></html>';

$to       = strip_tags($_POST['emailTo']);
$subject  = strip_tags($_POST['emailSubject']);
$headers  = "From: "     . strip_tags($_POST['emailFrom']) . "
";
$headers .= "Reply-To: " . strip_tags($_POST['emailFrom']) . "
";
$headers .= "MIME-Version: 1.0
";
$headers .= "Content-Type: text/html; charset=UTF-8
";

mail($to, $subject, $body, $headers);

header('Location: ' . $redirectTo);
?>