Very excited to be asking my first question on StackOverflow. I've been relying on it to teach myself quite a lot over the years!

My question is this. I am getting the following error when trying to send a mail through Mandrill's API:

```
{"status":"error","code":-1,"name":"ValidationError","message":"You must specify a key value"}

```

The code that follows is what I am using to try to send the mail:

```
<?php
$to = 'their@email.com';
$content = '<p>this is the emails html <a href="www.google.co.uk">content</a></p>';
$subject = 'this is the subject';
$from = 'my@email.com';

$uri = 'https://mandrillapp.com/api/1.0/messages/send.json';
$content_text = strip_tags($content);

$postString = '{
"key": "RR_3yTMxxxxxxxx_Pa7gQ",
"message": { 
 "html": "' . $content . '",
 "text": "' . $content_text . '",
 "subject": "' . $subject . '",
 "from_email": "' . $from . '",
 "from_name": "' . $from . '",
 "to": [
 {
 "email": "' . $to . '",
 "name": "' . $to . '"
 }
 ],
 "track_opens": true,
 "track_clicks": true,
 "auto_text": true,
 "url_strip_qs": true,
 "preserve_recipients": true
},
"async": false
}';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $uri);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true );
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postString);
$result = curl_exec($ch);
echo $result;

?>

```

What could be causing the validation error in the message. I am providing my API key, AND it's valid!

Hope someone will be able to help, and thanks for generally being AWESOME here!

Thanks!