To determine whether your configuration is working, send an email to the domain then check that it got published.
To check that your email was published:
Log in to the Google Cloud Console: https://console.cloud.google.com/.
Search for "pub/sub" in the top navigation bar and select the pub/sub result.
Select Subscriptions in the left hand navigation menu.
Select the subscription you created.
Click Push.
The push request data is displayed.