code: def init_app(self, app):
          '''Initialize the extension. Configuration will be 
          obtained from ``app.config['MARROWMAILER_CONFIG']``. If no 
          configuration is found the mailer will be configured to 
          send emails asynchrously via SMTP on localhost without 
          authentication. The created ``Mailer`` instance is written
          to ``app.marrowmailer``.'''
          if not hasattr(app, 'extensions'):
              app.extensions = {}
          mailer = BaseMailer(app.config.get('MARROWMAILER_CONFIG') or self.default_config)
          app.extensions['marrowmailer'] = mailer
          app.marrowmailer = self init_app(self, app):
          '''Initialize the extension. Configuration will be 
          obtained from ``app.config['MARROWMAILER_CONFIG']``. If no 
          configuration is found the mailer will be configured to 
          send emails asynchrously via SMTP on localhost without 
          authentication. The created ``Mailer`` instance is written
          to ``app.marrowmailer``.'''
          if not hasattr(app, 'extensions'):
              app.extensions = {}
          mailer = BaseMailer(app.config.get('MARROWMAILER_CONFIG') or self.default_config)
          app.extensions['marrowmailer'] = mailer
          app.marrowmailer = self
docs: Initialize the extension. Configuration will be 
          obtained from ``app.config['MARROWMAILER_CONFIG']``. If no 
          configuration is found the mailer will be configured to 
          send emails asynchrously via SMTP on localhost without 
          authentication. The created ``Mailer`` instance is written
          to ``app.marrowmailer``.