The instructions in the FAQ aren’t very clear. Here’s the way it would actually look in your wp-config.php file:
/* Mailgun Settings */
define('MAILGUN_USEAPI', '1'); //Set to 1 to override wp_mail and use Mailgun in WPMU (1 or 0)
define('MAILGUN_APIKEY', '<your api key here>'); //Found in your Mailgun dashboard
define('MAILGUN_DOMAIN', '<your domain>'); //The domain you setup in Mailgun
define('MAILGUN_USERNAME', '<Mailgun username>'); //The email address you setup in Mailgun
define('MAILGUN_PASSWORD', '<password>'); //The corresponding password for the email address in Mailgun
define('MAILGUN_SECURE', '1'); //If you want to use secure email or not (1 or 0)
(Don’t include the brackets < > in the above code btw)
Hope this helps!
kevin