GAE 发邮件总是说超额了

一个很简单的 cron 任务，一天也才发几封邮件而已，内容也绝对不算大的，就是几个K吧，但是经常说超标了，发送失败。

看过 app engine 的 Mail quota，应该远远在限额之下的，怎么会老是报错呢？
http://code.google.com/appengine/docs/quotas.html#Mail

错误如下：
500 11473ms 93cpu_ms 1kb AppEngine-Google; (+http://code.google.com/appengine)

File "/base/python_runtime/python_lib/versions/1/google/appengine/api/mail.py", line 796, in send
make_sync_call('mail', self._API_CALL, message, response)
File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 94, in MakeSyncCall
return stubmap.MakeSyncCall(service, call, request, response)
File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 308, in MakeSyncCall
rpc.CheckSuccess()
File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_rpc.py", line 133, in CheckSuccess
raise self.exception
OverQuotaError: The API call mail.Send() required more quota than is available.