def init_app(self, app):
if not hasattr(app, '<STR_LIT>'):<EOL><INDENT>app.extensions = {}<EOL><DEDENT>mailer = BaseMailer(app.config.get('<STR_LIT>') or self.default_config)<EOL>app.extensions['<STR_LIT>'] = mailer<EOL>app.marrowmailer = self<EOL>
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``.