This feature isn't available in the legacy free edition of Google Apps.
An email whitelist is a list of IP addresses from which your users expect to receive legitimate mail. When you add an IP address to your email whitelist, mails sent from this IP address will generally not be marked spam. To instead approve specific senders based on their email address or domain name, create an approved sender list using the Spam setting.
Please note that email whitelist is not exclusive. If you create a whitelist for your G Suite account, it will affect your entire domain. There's currently no functionality in place for you or your users to only receive mail from a self-defined list of senders.
After you configure an advanced Gmail setting, it may take up to one hour for that configuration to propagate to individual user accounts. You can track prior changes under Admin console audit log.
To add IP addresses to your email whitelist:
- Sign in to the Google Admin console.
- From the dashboard, go to Apps > G Suite > Gmail > Advanced settings.
- In the Organizations section, highlight your domain.
- In the Email whitelist section, enter the IP addresses of your contact's domain host to make sure any mail originating from these IP addresses are not labeled spam. If you would like to add more than one IP address, enter an IP range in CIDR notation or separate each IP address with a comma.
- Click Save changes.