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. This creates a confusing situation where the migration appears complete but mail routing continues using transitional paths.

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. This issue is particularly common in hybrid Exchange deployments where coexistence connectors were configured during migration but never decommissioned.

The problem frequently appears after hybrid migrations where dual-delivery was used during the transition, staged migrations where some users were moved before others, or when organizations used journaling, archiving, or compliance gateways that created delivery duplicates that were not removed after cutover.

Symptoms

  • Mail still splits between old and new environments after Microsoft 365 migration
  • Some users receive duplicate messages or messages in the wrong platform (on-prem vs cloud)
  • 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
  • Some mail goes to Exchange Online while copies still arrive on-premises
  • Journaling or archiving systems receive duplicate copies
  • Transport rules or connectors from migration still appear active
  • Mail flow reports show traffic to both old and new environments

Common Causes

  • An old dual-delivery or split-delivery path is still active for the domain in Exchange connectors
  • 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
  • Default remote domain settings still route mail to old environment
  • Journaling rules point to old on-premises journal targets
  • Third-party services configured during migration were not updated after cutover

Step-by-Step Fix

  1. 1.Capture message traces for the affected recipient in both Microsoft 365 and the old on-prem environment, and confirm whether mail is being delivered through more than one path, because you need proof of split routing before changing coexistence settings.
  2. 2.Check whether the domain still has any intentional dual-delivery, split-delivery, or transitional routing configuration in Exchange Online connectors and on-premises send/receive connectors, because one leftover coexistence rule can keep mail flowing to both environments after migration.
  3. 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 during post-migration cleanup.
  4. 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. 5.Disable or narrow the old duplication path at its real control point (connector, transport rule, journaling), because changing mailbox settings alone will not stop a legacy transport hop that still copies or reroutes messages.
  6. 6.Retest with the same sender and recipient combinations that exposed the issue, because split-delivery problems often affect only certain directions (internal vs external) or domains.
  7. 7.Review message 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. 8.Check whether related recipients or domains migrated in the same phase show similar duplication, because coexistence leftovers usually follow a pattern rather than affecting a single mailbox.
  9. 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.

Verification

Confirm the fix is complete:

  1. 1.Send test messages and verify they arrive only in Microsoft 365 mailboxes
  2. 2.Check message trace to confirm single delivery path
  3. 3.Verify on-premises mailboxes receive no new mail for migrated users
  4. 4.Test both internal and external senders
  5. 5.Monitor for 24 hours to ensure consistent behavior

Prevention

To avoid this issue in future migrations:

  • Create a coexistence cleanup checklist as part of migration planning
  • Remove dual-delivery connectors immediately after final cutover
  • Document all transitional routing components during migration setup
  • Include message trace validation in post-migration testing
  • Set reminders to decommission coexistence routing after migration
  • Review transport rules and connectors for migration-related entries
  • Audit journaling and archiving rules after migration completes
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