- you need to setup hostname correctly (both sides: Domain DNS and CWP settings)
- You need to add rDNS to your server/VPS (if the hosting provider allows you to do this, otherwise you need to ask them for help)
- You need to add SPF, DKIM to your server and your domain TXT records
- You need to generate, install, verify SSL on your server properly.
All these steps you can search google for tutorials, it's a long process. You might try to open email account at zoho.com (it is similar to Gmail for Work but allows you to add your own domain on free plan), then you will be able to read step by step on how to setup and verify DKIM + SPF