Use Mailgun as WordPress SMTP email sending

use-mailgun-as-wordpress-smtp.php

<?php
/** 
 * Use Mailgun as WordPress's default SMTP sender instead of built-in mailer.
 * All of these should be set for the best chance of email delivery.
 * Create constant MAILGUN_PASSWORD in wp-config.php for secure storage of API secret.
 */
function use_mailgun_as_smtp( $phpmailer ) {
    $phpmailer->isSMTP();
    $phpmailer->Host       = 'smtp.eu.mailgun.org'; // Or not if you're using NA
    $phpmailer->Port       = 587;
    $phpmailer->SMTPAuth   = true;
    $phpmailer->SMTPSecure = 'tls';
    $phpmailer->Username   = '[email protected]';
    $phpmailer->Password   = MAILGUN_PASSWORD;
    $phpmailer->From       = '[email protected]';
    $phpmailer->FromName   = 'Your Domain';
}
add_action( 'phpmailer_init', 'use_mailgun_as_smtp' );