curl_exec returns message but email is not sent

Question

I am using PHP version 7.1.9 with mailgun. Right now I am attempting to build the email portion of my code and I am using the following:

function send_email($from, $to, $subject, $body) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_setopt($ch, CURLOPT_USERPWD, 'api:[api found under domains > sandbox > API Key]');
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
    curl_setopt($ch, CURLOPT_URL, '[url found under domains > sandbox > API Base URL]');
    curl_setopt($ch, CURLOPT_POSTFIELDS, array(
        'from' => $from,
        'to' => $to,
        'subject' => $subject,
        'text' => $body
    ));

    $result = curl_exec($ch);

    curl_close($ch);
    return $result;
}

Whenever I execute the code the send_email function it returns the following String via the echo method:

Mailgun Magnificent API

Which to me, signifies that it successfully executed the function, however I never actually receive the email. Why would I get this echoed message, but not the email?

Some background information:

• Currently I am debugging locally via XAMPP v3.2.2 on my local host(127.0.0.1)
• As pointed out in the curl_setopt values, I am using the sandbox domain for testing purposes (I've masked them out in the code for obvious reasons)
• The email that I am sending to is verified in the Authorized Recipients section of the sandbox domain
• The Outgoing column under the sandbox domain confirms that 0 emails have been sent