composer.json

{
    "require": {
        "mandrill/mandrill": "1.0.34"
    }
}

Mandrill_inbound_to_outbound.php

<?php
 
require 'vendor/mandrill/mandrill/src/Mandrill.php';
 
define('API_KEY', '123456789QWERTY');
define('TO_EMAIL', '[email protected]');
define('TO_NAME', 'Foo Bar');
 
if(!isset($_POST['mandrill_events'])) {
    echo 'A mandrill error occurred: Invalid mandrill_events';
    exit;
}
$mail = array_pop(json_decode($_POST['mandrill_events']));
 
$attachments = array();
foreach ($mail->msg->attachments as $attachment) {
    $attachments[] = array(
        'type' => $attachment->type,
        'name' => $attachment->name,
        'content' => $attachment->content,
    );
}
 
$headers = array();
// Support only Reply-to header
if(isset($mail->msg->headers->{'Reply-to'})) {
    $headers[] = array('Reply-to' => $mail->msg->headers->{'Reply-to'});
}
 
try {
    $mandrill = new Mandrill(API_KEY);
    $message = array(
        'html' => $mail->msg->html,
        'text' => $mail->msg->text,
        'subject' => $mail->msg->subject,
        'from_email' => $mail->msg->from_email,
        'from_name' => $mail->msg->from_name,
        'to' => array(
            array(
                'email' => TO_EMAIL,
                'name' => TO_NAME,
            )
        ),
        'attachments' => $attachments,
        'headers' => $headers,
    );
    $async = false;
    $result = $mandrill->messages->send($message, $async);
    print_r($result);
} catch(Mandrill_Error $e) {
    // Mandrill errors are thrown as exceptions
    echo 'A mandrill error occurred: ' . get_class($e) . ' - ' . $e->getMessage();
    // A mandrill error occurred: Mandrill_PaymentRequired - This feature is only available for accounts with a positive balance.
    throw $e;
}