Use non Gmail/Gsuite on Gcloud projects
Small companies and startups use cheap email services or even Cpanel's mail services which are less secure and compete directly with bigger email providers like Microsoft with Outlook and Google with Gmail. This creates a problem when you try to use their services to send/receive emails from this cheap services.
<img src="https://static.techspot.com/fileshost/newspics3/2017/google-cloud-platform.png" />
Google says this:
[...]And that's it!