If you have the legacy free edition of G Suite, upgrade to G Suite Basic to get this feature.
As a G Suite administrator, you can help ensure that messages received from specific sending IP addresses don't get marked as spam. Do this by adding the addresses to an email whitelist in your Google Admin console.
When you create an email whitelist for your G Suite account, it affects your entire domain. You can’t create email whitelists that apply to specific organizational units. For more options, see Other settings you might use instead.
Add an IP address to your whitelist
From the Admin console Home page, go to AppsG SuiteGmailAdvanced settings.
Tip: To see Advanced settings, scroll to the bottom of the Gmail page.
On the left, select the top-level organization, typically your domain.
In the Spam, phishing, and malware section, scroll to the Email whitelist setting. Or, in the search field, enter email whitelist.
Enter the IP address of the sending mail servers you want to whitelist. To add more than one IP address, enter an IP range in Classless Inter-Domain Routing (CIDR) notation or separate each IP address with a comma.
Note: Be sure to enter public IP addresses, as private IP addresses are not accepted.
At the bottom of the Gmail Advanced settings page, click Save.
It can take up to 24 hours for any changes to take effect. You can track changes in the Admin audit log.
Spam setting—If you want to whitelist senders based on their email address or domain name, rather than their sending server IP address, create an approved senders list in the Spam setting.
Inbound gateway setting—If you have incoming mail servers that process your mail before sending to Google's servers, enter them in the Inbound Gateway setting instead. Remember, if you enter an IP address in an inbound gateway configuration and add the same IP address to an email whitelist, the IP address won’t be whitelisted. This is because of how Gmail determines the “true sender,” or source IP address.
Approved sender lists—In addition to the Spam setting, you can whitelist email addresses and domains using the address lists controls available in the Blocked senders, Restrict delivery, Secure transport (TLS) compliance, Content compliance, Attachment compliance, Objectionable content, and Routing settings.