As far as I can tell there is nothing wrong with this configuration. The only problem is that your application is not using it. You should update configuration before you initialize Mail
:
app = Flask(__name__)
app.config.update(dict(
DEBUG = True,
MAIL_SERVER = 'smtp.gmail.com',
MAIL_PORT = 587,
MAIL_USE_TLS = True,
MAIL_USE_SSL = False,
MAIL_USERNAME = 'my_username@gmail.com',
MAIL_PASSWORD = 'my_password',
))
mail = Mail(app)