As explained in this SendGrid documentation[1] there are ways to send email through SendGrid. I assume you are using the Web API, which requires authentication for each API call[2]. As explained in this documentation[3], these third party services use API keys to authenticate your app and you have to add the key to the environment variables section in your app's App.yaml file. This is a GitHub link[4] showing a Sample App.yaml file for an Application in App Engine Flex environment using SendGrid.
[1]https://sendgrid.com/blog/5-best-practices-for-using-sendgrid-with-google-app-engine/ [2]https://sendgrid.com/docs/API_Reference/Web_API/using_the_web_api.html#-Authentication [3]https://cloud.google.com/appengine/docs/flexible/php/sending-messages?hl=ru#adding_client_libraries_and_third-party_api_keys_to_your_app [4]https://github.com/GoogleCloudPlatform/php-docs-samples/blob/master/appengine/flexible/sendgrid/app.yaml -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/f1dcb203-b4d8-4d74-8218-1f5df5cc64a4%40googlegroups.com.
