Using PHP to access Gmail using Service Account - php

Question

I am trying to list emails from a standard Gmail account (not a Google Apps account) using OAuth2 and a Service Account (actually, I want to send emails – but that can wait).
I have created the Project, created the service account, downloaded the private key, and enabled the Gmail API (and the Calendar API). I can successfully access my calendar using code very similar to the code below. However, I am receiving the following error when attempting to list mail messages.
Error refreshing the OAuth2 token, message: error_description "Unauthorized client or scope in request."
(For info. If I comment out the $credential->sub = … line, I get the following error: “(500) Backend Error” and I can see that this is logged in the Usage tab of the Developers API page).
Can anyone confirm that what I am trying to do is possible? And if so, any ideas where to go next?
Here is my code:
class mail
{
private $service;
public function __construct($clientid, $keyfile, $account_name, $app_name)
{
$client = new Google_Client();
$client->setApplicationName($app_name);
$this->service = new Google_Service_Gmail($client);
// Load the key in PKCS 12 format
$key = file_get_contents($keyfile);
$client->setClientId($clientid);
$credentials = new Google_Auth_AssertionCredentials(
$account_name, array('https://mail.google.com/'),$key);
$credentials->sub = 'myemailaddress...#gmail.com';
$client->setAssertionCredentials($credentials);
}
public function list_mail()
{
try
{
$result = $this->service->users_messages->listUsersMessages("me");
}
catch (Exception $e)
{
throw new Exception("Gmail API error: ".$e->getMessage()."<br />");
}
return $result;
}
}
$mail_obj = new mail(CLIENT_ID, KEY_FILE, SERVICE_ACCOUNT_NAME, APP_NAME);
$result = $mail_obj->list_mail();