email - Sent Mail in PHP throw 504 Gateway Time-out

I'm on Ubuntu VM.

I have this PHP

#
            --------------------------------------------------------------------------------
#
            Goal : send an email
# Run : curl 45.55.88.57/code/mail.php |
            php


$to = 'email@gmail.com';
$subject =
            '';
$message = 'hello';

$headers = 'From: john@gmail.com'
            . "\r\n" .
 'Reply-To: john@gmail.com' . "\r\n" .
 'X-Mailer: PHP/'
            . phpversion();

mail($to, $subject, $message,
            $headers);
?>

/>

I ran this :

curl 45.55.88.57/code/mail.php |
            php

I get this

 % Total % Received % Xferd
            Average Speed Time Time Time Current
 Dload Upload Total Spent Left
            Speed
100 176 100 176 0 0 2 0 0:01:28 0:01:00 0:00:28
            45



            bgcolor="white">
504 Gateway
            Time-out
            />nginx

Is
my code is wrong, or is something wrong with my VM?

I don't get any
emails.