I am developing a microservice with PHP and appengine. I achieved to send emails with sendgrid. For security reasons the firewall was activate, but then the email service did not work enymore.
I know that the firewall its only for incoming traffic in this case, but that was the only thing that i change, and of course if i shut down the firewall the application works.
UPDATED
The appengiene microservice is call from a cron service.