app.config['MAIL_SERVER'] = 'smtp.googlemail.com'
app.config['MAIL_PORT'] = 587
app.config['MAIL_USE_TLS'] = True
app.config['MAIL_USERNAME'] = '[email protected]' # enter your email here
app.config['MAIL_DEFAULT_SENDER'] = '[email protected]' # enter your email here
app.config['MAIL_PASSWORD'] = 'password' # enter your password here
mail = Mail(app)
#...