我正在尝试异步发送一些邮件（基于Flask Mega-Tutorial，第十一部分：电子邮件支持）中的代码。但是，在应用程序上下文之外工作时，出现以下错误。我该如何解决这个问题？

Traceback (most recent call last):
  File "C:\Users\Primoz\Desktop\RecycleFlaskServer\recycleserver\helpers.py", line 17, in send_async_email
    mail.send(msg)
  File "C:\Python33\lib\site-packages\flask_mail-0.9.0-py3.3.egg\flask_mail.py", line 434, in send
    message.send(connection)
  File "C:\Python33\lib\site-packages\flask_mail-0.9.0-py3.3.egg\flask_mail.py", line 369, in send
    connection.send(self)
  File "C:\Python33\lib\site-packages\flask_mail-0.9.0-py3.3.egg\flask_mail.py", line 173, in send
    email_dispatched.send(message, app=current_app._get_current_object())
  File "C:\Python33\lib\site-packages\werkzeug-0.9.4-py3.3.egg\werkzeug\local.py", line 297, in _get_current_object
    return self.__local()
  File "C:\Python33\lib\site-packages\flask-0.10.1-py3.3.egg\flask\globals.py", line 34, in _find_app
    raise RuntimeError('working outside of application context')
RuntimeError: working outside of application context



from flask import Flask
from flask.ext.mail import Mail
app = Flask(__name__)
app.config.from_object('recycleserver.settings')
mail = Mail(app)

def async(f):
    def wrapper(*args, **kwargs):
        thr = Thread(target = f, args = args, kwargs = kwargs)
        thr.start()
    return wrapper

@async
def send_async_email(msg):
    mail.send(msg)

def send_simple_mail(subject, sender, to_who, text_body="", html_body=""):
    msg = Message(subject=subject,sender=sender, recipients=to_who)
    msg.body = text_body
    msg.html = html_body
    send_async_email(msg)