Sending email over SMTP with sendgrid

I have a html fieldset and a short php file for sending an email via sendgrid when the submit button is clicked.

HTML

            <form id="contact-form" action="scripts/mailer.php" method="post">
                <fieldset form="#contact-form">
                    <legend>Contact Form</legend>
                    <label class="input-field-name">Name:<br />
                        <input class="input-field" type="text" name="name" required/>
                    </label><br />
                    <label class="input-field-name">Email:<br />
                        <input class="input-field" type="text" name="email" required/>
                    </label><br />
                    <label class="input-field-name">Message Title:<br />
                        <input class="input-field" type="text" name="header" required/>
                    </label><br />
                    <label class="input-field-name">Message:<br />
                        <textarea class="message-field" type="text" name="message" required></textarea>                            
                    </label><br />
                    <button id="submit-button" type="submit">Submit</button>
                </fieldset>
            </form>

PHP

<?php
require_once('sendgrid-php/sendgrid-php.php');
$name = $_POST["name"];
$email = $_POST["email"];
$header = $_POST["header"];
$message = $_POST["message"];
$from = new SendGrid\Email(null, $email);
$subject = $header;
$to = new SendGrid\Email(null, "MY EMAIL ADDRESS");
$content = new SendGrid\Content("text/plain", $message);
$mail = new SendGrid\Mail($from, $subject, $to, $content);
$apiKey = 'MY API KEY';
$sg = new \SendGrid($apiKey);
$response = $sg->client->mail()->send()->post($mail);
if($response->statusCode() == 202){
    echo "Email sent successfully";
}else{
    echo "Email could not be sent";
}
echo $response->statusCode();
var_dump($response->headers());
echo $response->body(); 

I am hosting my site on GoDaddy and from everything I've been reading I believe that this should work but instead I get http error 500, my site cannot handle this request.

Any ideas of how to fix this? Thanks