Introduction

A Microsoft 365 migration can move users successfully while mail still splits across old and new environments because a dual-delivery path remains active. Messages may arrive in more than one mailbox, only some recipients may show duplicates, or certain flows may still pass through a legacy gateway that was meant only for coexistence.

Treat this as a coexistence-routing problem instead of a general mailbox-sync failure. Start by checking whether any old dual-delivery or split-delivery logic is still active for the affected domain, because partial migration routing can survive long after the main cutover appears complete.

Symptoms

  • Mail still splits between old and new environments after Microsoft 365 migration
  • Some users receive duplicate messages or messages in the wrong platform
  • External and internal senders see different delivery behavior for the same recipient
  • Message traces show multiple handoff paths for one migrated domain
  • The migration appears mostly complete, but coexistence-style routing still survives for part of the domain
  • The issue started after staged migration, hybrid transition, or mail-flow cleanup

Common Causes

  • An old dual-delivery or split-delivery path is still active for the domain
  • A legacy gateway, journal-like route, or coexistence transport rule still sends copies toward the old environment
  • Hybrid cleanup removed mailbox dependencies but left message-routing duplication behind
  • One connector, relay, or gateway still treats the domain as partially hosted elsewhere
  • Migration validation focused on successful delivery but missed duplicated or split delivery paths
  • Teams postponed coexistence cleanup and forgot one live routing dependency

Step-by-Step Fix

  1. Capture message traces for the affected recipient and confirm whether mail is being delivered through more than one path, because you need proof of split routing before changing coexistence settings.
  2. Check whether the domain still has any intentional dual-delivery, split-delivery, or transitional routing configuration, because one leftover coexistence rule can keep mail flowing to both environments after migration.
  3. Review connectors, gateways, relay services, and transport rules that were introduced during the migration, because duplicate delivery often survives in infrastructure that is no longer top of mind.
  4. Compare the affected recipients with the intended post-migration design, because you should only remove split routing after confirming the domain is meant to deliver solely through Microsoft 365.
  5. Disable or narrow the old duplication path at its real control point, because changing mailbox settings alone will not stop a legacy transport hop that still copies or reroutes messages.
  6. Retest with the same sender and recipient combinations that exposed the issue, because split-delivery problems often affect only certain directions or domains.
  7. Review traces before and after the change to confirm the extra handoff is gone, because one successful message does not prove the old dual-delivery path is fully removed.
  8. Check whether related recipients or domains migrated in the same phase show similar duplication, because coexistence leftovers usually follow a pattern rather than a single mailbox.
  9. Document the final mail-flow design and every transitional routing component that was removed, because dual-delivery logic is easy to leave behind during staged Microsoft 365 migrations.