php-mailparse composer not loading on server - php

Question

I have an Ubuntu php7.0 server and I am trying to use a php Mailparse script I found here
However I confirmed that composer is installed on the server and mailparse is also on the server. However the below script returns a 500 error and I tracked it down to the top two lines of code that is causing it and not sure how to resolve it.
So what I mean is when I comment out the two lines that say
require_once __DIR__.'/vendor/autoload.php';
$Parser = new PhpMimeMailParser\Parser();
Then the script will load but of course the mail parse wont work??
//load the php mime parse library
require_once __DIR__.'/vendor/autoload.php';
$Parser = new PhpMimeMailParser\Parser();
//Include the AWS SDK using the Composer autoloader.
require 'awssdk/aws-autoloader.php';
use Aws\S3\S3Client;
use Aws\S3\Exception\S3Exception;
// AWS Info
$bucketName = 'pipedemail';
$IAM_KEY = '******';
$IAM_SECRET = '******';
// Connect to AWS
try {
// You may need to change the region. It will say in the URL when the bucket is open
// and on creation. us-east-2 is Ohio, us-east-1 is North Virgina
$s3 = S3Client::factory(
array(
'credentials' => array(
'key' => $IAM_KEY,
'secret' => $IAM_SECRET
),
'version' => 'latest',
'region' => 'us-east-1'
)
);
} catch (Exception $e) {
// We use a die, so if this fails. It stops here. Typically this is a REST call so this would
// return a json object.
die("Error: " . $e->getMessage());
}
// Use the high-level iterators (returns ALL of your objects).
$objects = $s3->getIterator('ListObjects', array('Bucket' => $bucketName));
foreach ($objects as $object)
{
$objectkey = $object['Key'];
$path = "https://s3.amazonaws.com/pipedemail/$objectkey";
//lets get the raw email file to parse it
$Parser->setText(file_get_contents($path));
// Once we've indicated where to find the mail, we can parse out the data
$to = $Parser->getHeader('to'); // "test" <test#example.com>, "test2" <test2#example.com>
$addressesTo = $Parser->getAddresses('to'); //Return an array : [[test, test#example.com, false],[test2, test2#example.com, false]]
$from = $Parser->getHeader('from'); // "test" <test#example.com>
$addressesFrom = $Parser->getAddresses('from'); //Return an array : test, test#example.com, false
$subject = $Parser->getHeader('subject');
}