By default, Radius uses a built-in SMTP service. To help in avoiding problems with deliverability of your Auto Responders, Drip Campaigns and Individual Emails, using your own SMTP server is recommended.
If you are an Admin and setting the account SMTP, Go to Settings > EMAIL, COLUMNS, CALENDAR & OTHER SETTINGS
If you are an Agent and setting your own email SMTP, go to Account> My Info> Email
You'll see an option to Connect GMAIL Access. Click on that.
It will then walk you through the process.
Select the Email Address to sync by clicking on it.
and Grant Permission
It may tell you that This App Isn't Verified. If so, click on ADVANCED.
Click on Go to Radiusbob.com (Unsafe)
It will then Confirm that it has been set up and access is granted.
If you go back to your SMTP set up in Radius, you'll see that it's connected.