You have a Google Cloud server that can receive mail but cannot send mail.
Google Cloud intentionally blocks outbound mail on port 25.
Because Google Cloud blocks outbound mail, you must configure a smart host that can send mail on behalf of your server.
This is a common behavior among many cloud providers, we have more documentation about this specific practice here:
You can use one of the following guides to learn how to configure a smart host for your server so that you may send mail on Google Cloud servers: