Exchange Server 2016 and 2019 reached end of support on October 14, 2025. Microsoft's supportability matrix is the starting point for checking supported versions and dependencies. Exchange Server supportability matrix.

A migration plan needs more detail than a mailbox count. Inventory the installed builds, server roles, identity configuration, certificates, applications and mail connectors before selecting a destination. Record any separately purchased extended-update entitlement and confirm its actual scope and expiry with Microsoft; do not assume ordinary product support continues.

Choose the destination by requirements

Exchange Online moves mailbox hosting to Microsoft's cloud service. Evaluate licensing, tenant security, administration, retention and the applications that currently relay mail through your servers. Confirm the applicable service data-location commitments separately from Azure region announcements.

Exchange Server Subscription Edition (SE) is an on-premises option where requirements justify operating Exchange infrastructure. It still needs supported dependencies, licensing, patching, monitoring and recovery.

Hybrid operation may be part of a migration or an ongoing design. Establish which functions remain on premises, who operates them and how that arrangement will be supported. It should have an explicit purpose and ownership.

Check the upgrade path for the target build

Microsoft documented an in-place upgrade from Exchange 2019 CU14 or CU15 to Exchange SE RTM. That statement applies to the RTM release path; it is not permission to upgrade any Exchange version directly to any later SE build. Exchange 2016 requires a migration approach rather than that in-place upgrade. Microsoft Exchange SE RTM announcement.

Check the current supportability matrix and target-release documentation when scheduling the work. Include coexistence, operating system, Active Directory and client compatibility. Do not combine an Exchange change with an assumed supported operating-system upgrade without checking both products.

Test more than mailbox access

A pilot should cover mail flow in both directions, calendar and delegation behavior, mobile and desktop clients, shared mailboxes, archives and application relay. Review public folders and third-party integrations where used.

Verify DNS changes, certificates, authentication and connector settings. Check current Microsoft notices for your exact mail-flow configuration instead of relying on a universal throttling deadline or error code.

Prepare the cutover and recovery

  • Choose representative pilot users and obtain their acceptance.
  • Record migration batches, dependencies and the support contact for each stage.
  • Define the rollback decision and what remains recoverable after each change.
  • Communicate client changes and expected interruptions.
  • Remove old servers only after checking remaining dependencies and supported decommissioning instructions.

For older client devices, use the Windows 10 upgrade and ESU planning guide alongside email migration testing to identify endpoint dependencies.

BustanTech offers Microsoft 365 and managed services. Discuss a migration scope covering your installed versions, target platform and acceptance criteria.