Gmail API returns 403 error code and "Delegation denied for <user email>"

Question

Gmail API fails for one domain when retrieving messages with this error:

com.google.api.client.googleapis.json.GoogleJsonResponseException: 403 OK
{
  "code" : 403,
  "errors" : [ {
    "domain" : "global",
    "message" : "Delegation denied for <user email>",
    "reason" : "forbidden"
  } ],
  "message" : "Delegation denied for <user email>"
}

I am using OAuth 2.0 and Google Apps Domain-Wide delegation of authority to access the user data. The domain has granted data access rights to the application.

Answer 1

Seems like best thing to do is to just always have userId="me" in your requests. That tells the API to just use the authenticated user's mailbox--no need to rely on email addresses.