Set up LuxSci for only selected Email Addresses

Only Select Users or Groups

Create User Group

Login to your Google Workspace account as the account administrator.
Click Directory > Groups on the left-hand side.
Click Create group.
Create a name and email address for the group and then click Next.
Adjust the Access Type permission settings as needed. These settings can be changed later.
Click Create Group.
Click Add members to Group. You need to add every user that exists in your LuxSci account to the group. If more users need to be added later, they should be added to this group and as users in  LuxSci.com .
Click Add members. Enter as they appear in Google and LuxSci then click Add to Group.

Add Route

Click Apps > Google Workspace > Gmail on the left-hand side.
Click Hosts.
Click Add Route.
Enter the route's name, such as "LuxSci Secure Connector".
Under "Specify email server", choose Single host.
Enter LuxSci Server Name  email7401.luxsci.com  in the Host name or IP field and in the field to the right enter 25 for the port number.
Under "Options", check the boxes for Require secure transport (TLS), Require CA signed certificate, and Validate certificate hostname (Recommended).
Click Save.

Configure Your Routing and Assign to Your User Group

On the left-hand side, click Gmail to return to the main Gmail settings page.
Scroll down and click Routing.
To the right of the "Routing" section, click Configure.
Give your route configuration a name.
Under "Messages to affect", click Outbound.
Under "For the above types of messages, do the following", leave the default Modify Message selected.
Scroll down and check the box for Change Route.
From the "Normal Routing" dropdown menu, select the Route name you created earlier.
Scroll down and check the box for Encryption - Required secure transport (TLS). Then click Show options.
Under "Account types to affect", check Users and Groups.
Under "Envelope Filter", click Only affect specific envelope senders.
In the "Single email address" dropdown menu, click Group membership (only sent mail).
Click Select groups and add your Group created earlier.
Click Save.