protected function validate_addresses()
	{
		$failed = array();

		foreach (array('to', 'cc', 'bcc') as $list)
		{
			foreach ($this->{$list} as $recipient)
			{
				if ( ! filter_var($recipient['email'], FILTER_VALIDATE_EMAIL))
				{
					$failed[$list][] = $recipient;
				}
			}
		}

		if (count($failed) === 0)
		{
			return true;
		}

		return $failed;
	}