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



504 Gateway Time-out
nginx

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

I don't get any emails.