All help

Keep Google Workspace and add PostPike

Google calls it split delivery.

Add PostPike as a mail route

  1. In the Google Admin console, go to Menu, Apps, Google Workspace, Gmail, then Hosts, and select Add Route.
  2. Enter PostPike as the name. Under Specify email server, choose Single host, and enter mx1.postpike.net with port 25.
  3. Leave Perform MX lookup on host off. Keep the secure transport (TLS) and certificate options on, and select Save.

Pass unknown addresses on

  1. Go to Menu, Apps, Google Workspace, Gmail, then Routing. In the Routing section, select Configure or Add Another Rule.
  2. Enter a name, and check Inbound messages and Internal inbound messages.
  3. Under Action, choose Modify message, check Change route, and choose the PostPike route.
  4. Select Show more options. Check All inactive and unrecognized accounts, and leave Active user account and Group account unchecked. Then select Save.
Google says changes can take up to 24 hours, but they usually work sooner. Then select Test on your domain's page.

From Google: Send email to 2 email systems with split delivery, Google: Add mail servers for Gmail email routing. Menus change: if something here doesn't match what you see, tell us.