Exchange to Microsoft 365 Migration

When did Exchange 2016 and 2019 reach end of support?

Extended support for Exchange Server 2019 and Exchange Server 2016 ended on 14 October 2025, according to Microsoft’s lifecycle pages. Since then, no general security updates have been released for either version. An internet-facing mail server running without patches is exactly what attackers hope to find.

For organisations that could not finish migrating in time, Microsoft opened a separately purchased Extended Security Update (ESU) programme. According to the Period 2 announcement, this period runs from early May 2026 to the end of October 2026, and Microsoft states clearly that there will be no further extension. In other words, at the end of October 2026 the door closes on updates even for organisations that bought ESU.

For those who want to stay on premises, Microsoft’s route is Exchange Server Subscription Edition (SE). For most SMEs, though, running their own mail server makes far less sense than it used to; moving to Exchange Online hands the work of backups, patching, certificates and spam filtering over to Microsoft.

Cutover, staged or hybrid: which Exchange migration method should you use?

Microsoft’s page on migration methods describes three main routes. A cutover migration moves every mailbox in one go; Microsoft positions it for older versions and small numbers of mailboxes, and says that despite the 2,000 limit, staying under 150 users is more reasonable. A staged migration is defined only for Exchange 2003 and 2007, so in practice it no longer comes up.

For Exchange 2013 and later, Microsoft’s recommended route is hybrid. The Hybrid Configuration Wizard links on-premises Exchange and Exchange Online; mailboxes move in batches, and migrated and not-yet-migrated users can still see each other’s calendar free/busy information. Even if the migration takes weeks, users experience it as a single mail system.

For smaller, shorter projects there is minimal hybrid (Express migration): it is meant for organisations running Exchange 2010, 2013 or 2016 that want to move within a few weeks and will not keep directory sync running. We decide which option suits you by looking at the number of mailboxes, total data size, the future of your Active Directory and the upload speed of your internet connection.

How do you plan the mail flow and MX cutover?

Mail flow is the most delicate moment of the migration. A few days beforehand we lower the TTL on the MX and Autodiscover records, verify the domain in Exchange Online, add Microsoft 365 to the SPF record and prepare DKIM signing. The moment the MX changes, new mail starts landing in the cloud; the few messages already on their way to the old server still reach their destination over the hybrid connection.

Devices that send mail over SMTP, such as scanners, photocopiers or the ERP system, are the items most likely to be forgotten. We list them in advance and reconfigure each one with the right method: direct send to Exchange Online, SMTP relay or a connector. After the migration you can tighten SPF, DKIM and DMARC with our email security work.

What happens to shared mailboxes and public folders?

In a hybrid migration, shared mailboxes (accounts@, sales@ and the like) move just like ordinary mailboxes and carry on working as shared mailboxes in Exchange Online. The thing to watch is that the users who access a shared mailbox move in the same batch as the mailbox itself; otherwise Send As and Full Access permissions can misbehave for a while.

Public folders move through a separate batch migration, usually once the mailboxes are done. In some companies nobody has opened them in years; in others, the firm’s entire correspondence history lives there. We measure how they are used first. If they are not needed, we discuss archiving them and leaving them behind; if they are, we discuss converting them into Microsoft 365 groups or shared mailboxes.

Can you migrate Exchange without downtime?

In a hybrid migration, the only interruption a user sees is restarting Outlook once after their own mailbox’s final sync. Mailboxes copy in the background for weeks; we run the final completion step in the evening and the user starts the next morning in their new mailbox.

Picture a logistics company with 120 users on Exchange 2019. In week one, Entra Connect and hybrid are set up and the IT team’s mailboxes move as a pilot. Over the next two weeks the company moves department by department, with finance left until last so it does not clash with month-end close. The MX record changes at the very end, and the on-premises Exchange server stays on for a while for management purposes only.

After the migration you also need to think about backing up your Exchange Online data; Microsoft’s retention policies are not a substitute for a backup product. For Microsoft 365 backup, see our Veeam backup page.

• Pre-checks: Exchange version and CU, certificates, Autodiscover, DNS
• Identity: Entra Connect or Cloud Sync, UPN fixes
• Pilot batch and user feedback
• Department batches, completed in the evening
• MX, SPF and DKIM cutover, and reconfiguring SMTP devices

What you receive

• Inventory of the Exchange environment and a decision on migration method
• Hybrid or minimal hybrid configuration and identity sync
• Migration of mailboxes, shared mailboxes and public folders
• Cutover of MX, Autodiscover, SPF and DKIM records
• Reconfiguration of devices and applications that send over SMTP
• A short announcement for users and a post-migration check report

How we work

1. 1

   Free review

   We list the Exchange version, mailbox count and sizes, public folders, certificates and devices that use SMTP.

2. 2

   Plan

   You receive the migration method, batch order, MX window and rollback steps in writing.

3. 3

   Preparation

   The Microsoft 365 tenant, domain verification, identity sync and hybrid connection are set up.

4. 4

   Migration

   A pilot batch moves first, then department batches, with user feedback collected after each one.

5. 5

   Close-out

   The MX record switches, devices are reconfigured and the retirement of the old server is planned.

Frequently asked questions