leaky-paywall-auth-webhook-event.php

<?php 

// Send an email to the subscriber when Authorize.net sends a net.authorize.customer.subscription.expired webhook event
// Notice the action is the event name, with the periods changed out for underscores
// Any event can be accessed that way

add_action('leaky_paywall_net_authorize_customer_subscription_expired', 'zeen_send_expired_notice', 10, 2);

function zeen_send_expired_notice($user, $webhook_data)
{
    $to = $user->user_email;
    $subject = 'Your subscription has expired';
    $body = 'Your subscription has expired.  Please renew here:  https://example.com';
    $headers = array('Content-Type: text/html; charset=UTF-8', 'From: My Site Name <[email protected]');

    wp_mail($to, $subject, $body, $headers);
}