When ImprovMX receives an email for a domain but no alias matches the recipient address, the email currently bounces. With split delivery, instead of bouncing, ImprovMX would relay that email to a fallback mail server configured by the customer.

This is the same concept as split delivery / dual delivery in Google Workspace and Microsoft 365. It's especially useful for enterprise customers migrating large numbers of aliases from another email provider (e.g. Rackspace, Exchange). Instead of migrating all aliases at once, customers can move them over in waves — any address not yet migrated in ImprovMX automatically falls through to their old mail server, so no email is lost during the transition.

Beyond migrations, this could also serve as a general catch-all relay, where customers who use ImprovMX for some addresses can route everything else to their primary mail server.