Why do older Exchange servers need to move now?
Exchange 2010, 2013, 2016 and 2019 are all out of support. Exchange 2016 and 2019 left support on 14 October 2025, and Microsoft’s paid Extended Security Update (ESU) program for them ends on 31 October 2026. ESU covers security fixes only, on Exchange 2016 CU23 or Exchange 2019 CU14/CU15. After it ends, there are no more patches.
Nothing breaks visibly the next day, but Exchange Online throttles and can block mail from persistently vulnerable on-premises servers, so an unpatched server becomes a delivery problem as well as a security one.
You have two supported destinations: upgrade to Exchange Server Subscription Edition (SE), which is in-place only from Exchange 2019 CU14 or CU15, or move to Exchange Online in Microsoft 365. This guide covers the move to the cloud service.
The discussion of outdated server infrastructure and its vulnerabilities extends beyond digital security. In architectural design, the physical housing of such servers—whether in dedicated data centers or on-site server rooms—demands careful consideration of space, cooling, and energy efficiency. Modern IT strategies, like cloud migration, often align with sustainable design principles by reducing the physical footprint and energy demands of on-premises hardware.
What should you inventory before migrating?
Every successful Exchange to Microsoft 365 migration starts with an audit. Record:
- Mailbox count and size, archive mailboxes, and PST files on laptops and shares
- Shared, room and equipment mailboxes, distribution groups and public folders
- Mailbox permissions (Full Access, Send As, Send on Behalf) and calendar delegates
- Your exact Exchange version and cumulative update, transport rules, connectors and journaling
- Every device and application that relays mail through Exchange: scanners, ERP systems, monitoring alerts
Relays are what most audits miss. This command lists anonymous relay connectors and the IPs using them:
Get-ReceiveConnector | Where-Object { $_.PermissionGroups -match ‘AnonymousUsers’ } | Format-Table Name, Bindings, RemoteIPRanges -AutoSize
Also check your largest mailboxes against license limits. Microsoft 365 Business Basic, Standard and Premium and Microsoft 365 E3/E5 give a 100 GB primary mailbox; Office 365 E1 gives 50 GB, and F3 only 2 GB. Unlicensed shared mailboxes are capped at 50 GB. Microsoft recommends keeping each public folder under 25 GB.
Which migration method fits your Exchange version?
| Source version | Under 150 mailboxes | 150 or more | Supported hybrid versions |
| Exchange 2010 | Cutover or express | Hybrid via a newer server, or a third-party tool | 2010, 2013 or 2016-based (not 2019) |
| Exchange 2013 | Cutover or express | Hybrid, often via a newer server | 2013, 2016 or 2019-based |
| Exchange 2016 | Cutover or express | Hybrid | 2016 or 2019-based |
| Exchange 2019 | Cutover or express | Hybrid | 2019-based |
What to know about each method:
- Cutover: moves everyone in one go. Supported up to 2,000 mailboxes, but Microsoft recommends about 150 or fewer. Users arrive as new cloud users and still need licenses.
- Express (minimal hybrid): a quick one-time move for small organisations.
- Hybrid: larger moves in waves while both sides coexist. Needs the Hybrid Configuration Wizard, certificates and directory sync; can run for months.
- IMAP: copies email only, not contacts, calendars or tasks. Use it as a last resort.
- Third-party tool: for large, cross-forest or reporting-heavy projects, at a license cost.
How do you run the migration without disrupting users?
- Pilot first. Move 10 to 20 people from different departments and fix what breaks.
- Move in waves around the business. Avoid finance at month-end, and tell each group when their mailbox moves and who to call.
- Protect the data. Export mailbox permissions and delegates, record retention policies and litigation holds (they must be recreated in Microsoft Purview), and use dedicated admin accounts with MFA.
- License users before their mailboxes move.
Ensuring seamless IT transitions, as described here, is paramount for maintaining productivity within any designed workspace, from a bustling open-plan office to a focused design studio. The reliability of digital tools and access to data directly impacts how effectively architects and designers can collaborate, innovate, and bring their visions to life, making the underlying infrastructure a critical component of the overall user experience within a built environment.
What is on the cutover-day mail flow checklist?
- Lower the MX record TTL a few days ahead.
- Point MX and Autodiscover to Microsoft 365 in both internal and external DNS.
- Clear the Autodiscover service connection point (SCP) in Active Directory, which domain-joined Outlook checks before DNS.
- Update SPF and set up DKIM and DMARC.
- Repoint every scanner and application that relays through the old server.
- Test how Outlook profiles and mobile devices reconnect during the pilot.
When does a migration tool help?
If coordinating this by hand is too much for your team, a dedicated Exchange migration tool such as Stellar Migrator for Exchange is worth evaluating. The vendor makes this tool, so compare it with the native options on your own requirements.
- Sources: Exchange 2010 to 2019 and Exchange SE, with Microsoft 365 as the destination
- Automatic, manual or CSV bulk mailbox mapping
- Up to 10 mailboxes in parallel, with scheduled jobs
- Incremental runs that pick up only new or failed items
- Primary, archive, shared and public folder mailboxes
- Cross-forest and cross-domain moves without an AD trust
How do you know the migration worked?
- Item counts and folder structures match between source and destination.
- Shared mailboxes open for the right people and Send As works.
- Every error and skipped item in the reports is explained or fixed.
- Mail flows both ways, including from scanners and applications.
How do you retire the last Exchange server?
Do not uninstall your last Exchange server just because the last mailbox moved. If you sync users from on-premises Active Directory, most Exchange attributes can only be edited with on-premises Exchange tools. Microsoft calls this the last Exchange server problem. You have three options:
- Keep one server for recipient management. The simplest choice, but the server must stay patched and licensed.
- Use Exchange Management Tools only. Manage recipients through PowerShell with the management tools role, then shut the server down. Do not uninstall it: the Exchange objects in Active Directory must remain. Keep the server if it still relays mail.
- Move Exchange attributes to the cloud, then uninstall. Transfer the source of authority for Exchange attributes to Exchange Online, then follow Microsoft’s steps to uninstall the last server.
Do this in order. Transfer source of authority only after every on-premises mailbox has migrated and you no longer create recipients on-premises. Microsoft warns that doing it earlier is unsupported and can block remaining mailbox moves.
Frequently asked questions
Can I migrate Exchange 2010 directly to Microsoft 365? Yes, with cutover or express for small organisations. Larger hybrid moves need a 2016-based or older hybrid server, since Exchange 2010 cannot use a 2019-based hybrid.
Do users need new Outlook profiles? Not with hybrid moves, because the mailbox identity stays the same. Cutover and IMAP moves usually require profile reconnection.
Final thoughts
With ESU ending on 31 October 2026, the timing is hard to argue with. Inventory the relays, match the method to your version, pilot, cut over carefully, validate, and then retire the last server by the path that fits your setup.
