To route unknown g suite users to another email server

Host Settings

Log in to Admin Console
Click on Apps >> Gsuite >> Gmail >> Advance Settings
Create a Host

By default the host record will be your custom MX record or

Routing Settings: 

Once the host details are saved, we need to proceed with adding the routing settings. 

Click on Apps >> 

Gsuite >> 

Gmail >> 

Advanced Settings

Under General settings >> 

Scroll down to Routing >> 

Besides Routing >> 

Click on Configure.

Route >>
Click on Change Route >>
Select the Hostname that was created in the previous step.
The reference hostname was  “Business email routing”.

Scroll down >>
Click on Show Options >>
Account types to affect >> Select Unrecognized / Catch -all

Click on Add Settings.

This completes the split delivery setup for your domain name. Going forward if any email is sent, the email will be first routed to your Gsuite. If the account exists under Gsuite the mail will be delivered successfully.

In case the email account does not exist, the mail will be routed to your Business/ Enterprise Email server and the message will be delivered to the intended recipient.