I have upgraded a site to Laravel 5.5 and suddenly I can no longer send mail, as it gives me the following error:

> 
> production.ERROR: Expected response code 250 but got code "550", with
>  message "550-Not authenticated, please enable SMTP Authentication in
>  email software and 550 check login credentials
> 
> 
> 

I have no idea why it is trying to use SMTP when I have specified mail in my `.env`, which it seemingly ignores:

```
MAIL_DRIVER=mail
MAIL_HOST=
MAIL_PORT=25
MAIL_USERNAME=null
MAIL_PASSWORD=null
MAIL_ENCRYPTION=null

```

This is a completely fresh installation of Laravel 5.5. I have run `composer dumpautoload` and `artisan cache:clear`.

Help?

**EDIT:**

A simple PHP file with the following code works fine:

```
$to      = 'my_email_address@hotmail.com';
$subject = 'the subject';
$message = 'hello';
$headers = 'From: webmaster@example.com' . "\r\n" .
    'Reply-To: webmaster@example.com' . "\r\n" .
    'X-Mailer: PHP/' . phpversion();

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

exit();

```