Follow this instructions to integrate SMTP with Gmail API: 

• 1. Go to the Google Auth Platform.
• 2. Create a new project or select existing project. 

• 3. Select your project and click on GET STARTED button to configure Google Auth Platform.

• 4. Complete the steps.

• 5. Click Audience from left menu and 
  publish the app.

After publishing, the status should be In production. 

• 6. Go to  API Library and search for Gmail API and select that.

• 7. Enable the Gmail API.

• 8. Go to  Google Auth Platform again and click on Clients from left menu and CREATE CLIENT.

Copy the Authorized redirect URI from mailbox's outgoing email settings, follow the below screenshot and paste the URI in the URIs input box. 

• 9. Collect the Client ID and Client secret. 

Great! You are almost ready. Go back to the RISE. 

• 10. Save the Client ID and Client secret in the mailbox's outgoing email settings. 

• 11. You'll be redirected to Google hasn't verified this app page.  Click on the Advanced link. 

Note: To remove this warning message, please click on the Learn more link. There is a process to verify your app. That's a different part and don't have anything to do in RISE. 

• 12. Click on the Go to yourdomain.com (unsafe) link. 

• 13. Sign in with Google and continue. Then you'll have to allow this permissions. Allow them all and continue. 

• 14. You'll be redirected to the mailboxes page. The integration status should be Authorized. 

Congratulations! All done. Now you can send email through Gmail API.