relay_via_mailgun.js

var mailgun_smtp = {
    auth_user: 'xxx',
    auth_pass: 'yyy',
    priority: 0,
    exchange: 'smtp.mailgun.org',
};

exports.hook_get_mx = function (next, hmail, domain) {
    // All relaying goes via mailgun
    return next(OK, mailgun_smtp);
}