Am finding that the Email Settings API is throwing random 403 errors.
I am using the gdata python API on App Engine. My app spawns a new task for each user in a Google Apps domain, then each task calls the retrieve_signature method. Sometimes this call throws a 403 error. The task will then auto retry. Eventually all tasks will complete. We have noticed that this only tends to happen with larger domains (> 100 users).
An obvious explanation for this, would be we are firing too many requests in too little time and therefore exceeding some kind of hidden quota, but it doesn't make sense that the error should be a 403.