How to Configure Dual Delivery Email Routing in Google Workspace

The Migration Challenge

Migrating a 5,000-employee corporation from an archaic on-premise Microsoft Exchange server to Google Workspace is never done overnight. It is a phased approach that can take months. During this transition period, half of the company (e.g., the Sales department) will have their active inboxes on Google Workspace, while the other half (e.g., Engineering) remains on the legacy Exchange server.

This creates a massive routing dilemma: If the MX (Mail Exchanger) records point to Google, how do emails reach the engineers still on Exchange? If the MX records point to Exchange, how do the sales reps on Google get their mail?

The architectural solution is Dual Delivery (sometimes called Split Delivery, though they operate slightly differently). In a true Dual Delivery setup, the MX records point exclusively to Google Workspace. Google receives every incoming email. If the recipient is a Sales rep, Google delivers it to their Gmail inbox. If the recipient is an Engineer, Google accepts the email, realizes the user doesn’t have a Gmail inbox yet, and securely forwards (routes) the email down to the legacy Exchange server.

Step 1: Adding the Legacy Server as a Host

Before you can instruct Google to forward emails to your Exchange server, you must define the Exchange server as a trusted destination.

1. Log into the Google Workspace Admin Console.
2. Navigate to Apps > Google Workspace > Gmail > Hosts.
3. Click Add Route.
4. Name the route (e.g., Legacy Exchange Server).
5. Specify the public IP address or FQDN of your Exchange server (e.g., mail.corp.com).
6. Ensure Require TLS is checked to prevent plaintext transmission over the internet, and click Save.

Step 2: Configuring the Routing Policy

Now you must create the logical rule that dictates exactly which emails are sent to that host.

1. Navigate to Apps > Google Workspace > Gmail > Routing.
2. Scroll to the Routing section and click Configure.
3. Name the rule (e.g., Dual Delivery to Exchange).

Step 3: Defining the Delivery Logic

This is where the precision matters to ensure you do not create infinite mail loops.

1. Email messages to affect: Select Inbound and Internal – receiving.
2. Scroll down to Step 2 (“For the above types of messages, do the following”).
3. Under the Route section, check the box for Change route.
4. From the dropdown, select the Legacy Exchange Server host you created in Step 1.

Step 4: Applying the Unrecognized Address Filter

If you save the rule right now, Google will forward every single email to Exchange, ignoring its own Gmail inboxes. You must apply a conditional filter.

1. Scroll down to Step 3 (“Options”).
2. Select Only affect specific envelope recipients.
3. Change the dropdown to Unrecognized / Catch-all.
4. Click Save.

How the Logic Works

With this specific configuration, the flow is flawless:

• An external vendor sends an email to [email protected]. Google receives it, checks its directory, sees that the Sales Rep has an active Workspace license and Gmail inbox, and delivers it locally. The routing rule is completely ignored.
• An external vendor sends an email to [email protected]. Google receives it, checks its directory, and realizes [email protected] does not have an active Gmail inbox (they are “Unrecognized”). Google’s routing rule immediately triggers, intercepts the message, and fires it over TLS directly to the on-premise Exchange server for final delivery.

This ensures seamless communication during a multi-month migration without dropping a single message.