Using Flask-Mail asynchronously results in "RuntimeError: working outside of application context"

I am trying to send some mail asynchronously (based on the code in [The Flask Mega-Tutorial, Part XI: Email Support][1]). However, I get the following error about working outside an application context. How do I fix this problem?

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

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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)

[1]:

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The code should run in an app context. Add `with app.app_context()`:

@async
def send_async_email(msg):
with app.app_context():
mail.send(msg)