¿Cómo puedo evitar el permiso insuficiente HttpError 403? (API de Gmail, Python)

Sigo recibiendo el siguiente error cuando ejecuto mi código:

An error occurred: <HttpError 403 when requesting https://www.googleapis.com/gmail/v1/users/me/messages/send?alt=json returned "Insufficient Permission">

Este es mi código:

import httplib2
import os
from httplib2 import Http

from apiclient import discovery
import oauth2client
from oauth2client import client
from oauth2client import tools

try:
    import argparse
    flags = argparse.ArgumentParser(parents=[tools.argparser]).parse_args()
except ImportError:
    flags = None

#SCOPES = 'https://www.googleapis.com/'
SCOPES = 'https://www.googleapis.com/auth/gmail.compose'
CLIENT_SECRET_FILE = 'client_secret.json'
APPLICATION_NAME = 'Gmail API Quickstart'

def get_credentials():
    """Gets valid user credentials from storage.

    If nothing has been stored, or if the stored credentials are invalid,
    the OAuth2 flow is completed to obtain the new credentials.

    Returns:
        Credentials, the obtained credential.
    """
    home_dir = os.path.expanduser('~')
    credential_dir = os.path.join(home_dir, '.credentials')
    if not os.path.exists(credential_dir):
        os.makedirs(credential_dir)
    credential_path = os.path.join(credential_dir,
                                   'gmail-quickstart.json')

    store = oauth2client.file.Storage(credential_path)
    credentials = store.get()
    if not credentials or credentials.invalid:
        flow = client.flow_from_clientsecrets(CLIENT_SECRET_FILE, SCOPES)
        flow.user_agent = APPLICATION_NAME
        if flags:
            credentials = tools.run_flow(flow, store, flags)
        else: # Needed only for compatability with Python 2.6
            credentials = tools.run(flow, store)
        print 'Storing credentials to ' + credential_path
    return credentials

def CreateMessage(sender, to, subject, message_text):
  """Create a message for an email.

  Args:
    sender: Email address of the sender.
    to: Email address of the receiver.
    subject: The subject of the email message.
    message_text: The text of the email message.

  Returns:
    An object containing a base64 encoded email object.
  """
  message = MIMEText(message_text)
  message['to'] = to
  message['from'] = sender
  message['subject'] = subject
  return {'raw': base64.b64encode(message.as_string())}

testMessage = CreateMessage('ENTER SENDERS EMAIL ADDRESS', 'ENTER RECEIVERRS EMAIL ADDRESS', 'ENTER SUBJECT', 'ENTER EMAIL BODY')

def SendMessage(service, user_id, message):
  """Send an email message.

  Args:
    service: Authorized Gmail API service instance.
    user_id: User's email address. The special value "me"
    can be used to indicate the authenticated user.
    message: Message to be sent.

  Returns:
    Sent Message.
  """
  try:
    message = (service.users().messages().send(userId=user_id, body=message)
               .execute())
    print 'Message Id: %s' % message['id']
    return message
  except errors.HttpError, error:
    print 'An error occurred: %s' % error


testSend = SendMessage(service, 'me', testMessage)

Sigo leyendo que necesito editar un archivo de credenciales, pero parece que no puedo encontrarlo. Tengo instalado Windows 7. ¿Alguien sabe lo que debo hacer para superar este error? Soy un novato total en esto, así que discúlpeme si parezco un poco novato al respecto. ¡Gracias!