Migration Exchange — Microsoft 365 and Exchange Server SE
Migrate Exchange without interrupting email service.
Exchange Server, Exchange Online, migration from one provider to another, or third-party email services. We plan, implement, and validate your migration while keeping the service up and running. Mailboxes, archives, calendars, delegations, domains, and email flows are all part of a strategy tailored to your environment—not to a catalog.
Exchange Server 2016 and 2019 have been out of support since October 14, 2025. The second and final extended security update period ends in late October 2026. After that date, no further security patches will be released for these versions, at any price.
The Dates That Matter
Nothing stops, nothing warns us. That's the problem.
A Exchange 2016 version that is no longer supported continues to deliver mail just as it did the day before. It simply no longer receives security patches, bug fixes, or support—and no one notices until an incident occurs. Here is the actual timeline, as published by Microsoft.
July1, 2025
Exchange Server SE is coming
The new on-premises version is available. It follows a modern lifecycle: no more versions to purchase every three years, and no announced end-of-support date as long as it is kept up to date.
October 14, 2025
End of support in 2016 and 2019
The two most widely deployed versions are reaching end of support. No more security patches, no more fixes, and no more support from Microsoft.
May → October 2026
The final grace period
The second period of extended security updates—which are paid and must be purchased— ends at the end of October 2026. After that, no further updates are scheduled for 2016 and 2019.
Second half of 2026
The SE CU2 Update
Its installer will not run alongside versions 2013, 2016, and 2019. The old servers must be removed from the organization before installing it.
Three options remain: Microsoft 365Exchange Server SE, or a hybrid combination of the two. We analyze the existing environment and determine which one is compatible with your applications, your security requirements, and your cloud — not the one that suits the vendor.
See Exchange Server SEThe Starting Point
What migration are you preparing?
Five scenarios, five methods, five timelines. The first question in a messaging project isn’t “which tool?”—it’s where we’re starting from and where we’re going.
Exchange Server toward Microsoft 365
Migrate mailboxes, archives, and messaging services to Exchange Online to modernize the environment and reduce on-premises infrastructure.
Our Page Migration Microsoft 365All in one piece Microsoft 365 from one to the next
Migrate users and data to another tenant during a merger, acquisition, spin-off, or corporate reorganization.
Our page Tenant to TenantThird-party messaging to Microsoft 365
Migrate from Google Workspace, an IMAP platform, a Swiss email provider, or another email solution to the Microsoft 365.
Migration toward a hybrid environment
Connect Exchange Online with an on-premises infrastructure when certain data, applications, or functions must remain on-site. It is also, very often, a transitional step in a migration prudent approach.
Our Hybrid page and CloudModernization ofExchange Server
Migrating a local infrastructure tExchange Server Subscription Edition when organizational constraints do not allow for a migration full migration to the cloud.
The section on Exchange Server SEThe version no one talks about
Exchange Server SE: Stay on-site, but use a supported version.
The cloud isn’t the only answer, and claiming otherwise does no one any favors. Since July 2025, Exchange Server the Subscription Editionhas been available: a subscription-based version with a modern lifecycle. It remains supported as long as it is kept up to date, with no announced end date.
For an organization that must keep its email system on-premises—whether due to data localization requirements, regulatory constraints, a business application that cannot be separated from the server, or an isolated network—this is currently the only way to have both an on-premises solution and technical support. All other versions are no longer supported or will be discontinued before the end of the year.
What’s really changing: there are no longer major versions to buy every three years, and no more projects to migration to replay every time. There are cumulative updates to install regularly, and a subscription to maintain. In exchange, Microsoft has committed to supporting its server products, Exchange including these, at least through 2035.
Microsoft is also placing Exchange Server SE, along with SharePoint and Skype for Business SE, at the heart of Microsoft 365 Local: Microsoft productivity running on infrastructure owned by the customer, either online or completely offline. This is a topic in its own right, and it applies to organizations whose data cannot leave the country.
How do we get there, according to your version?
- 2019 CU14/15 In-place upgrade Exchange Server SE is installed on top of the existing system, like a cumulative update. The server remains the same, and the mailboxes are not moved. This is the least risky and fastest approach.
- 2016 No on-site upgrade We need new servers, to migrate the mailboxes, and then to decommission the old environment. This is a migration — even if you stay on-site.
- 2013 or earlier Upgrading from an intermediate version Or, very often, the opportunity to choose Exchange Online rather than rebuilding an infrastructure that will need to be overhauled in two years.
- Already migrated The last server also counts An organization synchronized with a local directory typically maintains a server Exchange — or at least its management tools — to administer email attributes. It, too, must be running a supported version.
The stumbling block: the installation program for the SE CU2 update, expected in the second half of 2026, does not support coexistence with Exchange 2013, 2016, and 2019. Decommissioning the older servers is therefore not a convenience task to be put off “until later”—it’s a requirement for staying up to date.
The actual scope
A migration is not limited to mailboxes.
Emails are only part of the environment Exchange. Calendars, archives, delegations, shared folders, distribution lists, and public folders must also be inventoried and transferred. And most importantly: all systems that use Exchange to send messages —scanners, ERP, business applications, and monitoring platforms.
| What Else Needs to Be Migrated | Why It Matters | The Trap |
|---|---|---|
| Calendars, delegations, shared folders | An assistant’s permissions on a director’s calendar, service mailboxes, distribution lists: these are the invisible fabric of the organization. | The Trap: Noneof this shows up in a mailbox inventory. A missing delegation is discovered on Monday morning—by the director. |
| Archives and Public Records | These are often the largest volumes, the oldest ones, and those whose contents no one remembers anymore. | The Pitfall:Public folders have no direct equivalent. You have to decide, on a folder-by-folder basis, which ones will become shared folders, groups, or sites SharePoint. This decision is made in collaboration with the business units, not by the IT department alone. |
| Domains, DNS , and email feeds | MX records, SPF, DKIM, DMARC, filtering gateways, outgoing relays, and connectors. | The Pitfall: An incomplete SPF record will send your messages to the spam folder for all your clients—without triggering any alerts on your end. |
| Apps that send messages | ERP, business applications, multifunction scanners, alert systems, monitoring, and automated billing systems. | The Catch:Basic authenticationhas been phased out, and the EWS interface forExchange Online will begin to be disabled in October 2026, with full discontinuation in April 2027. Any application still using it will stop working. On Exchange Server, it remains available: sometimes it’s this detail that determines the outcome. |
| Identities, Licenses, and Directory | Accounts, groups, directory synchronization, service accounts, email attributes. | The Pitfall: A forgotten service account blocks the switchover for an entire department at the last minute, on a Friday night. |
| Landlines and Cell Phones | Outlook and its profiles, local rules, data files, managed terminals, and unmanaged terminals. | The Pitfall: Manually recreating a profile across 400 workstations is not just a minor detail—it’s a project, and its cost must be estimated before the migration. |
These dependencies are identified before the switchover. This is the only way to maintain email flow and restore all services in the new environment—rather than discovering them one by one in production while the phone is ringing.
Our Approach
A migration prepared, tested, and then validated.
Five steps, in this order, without skipping any. Each step produces something you keep: a document, a configuration, a measurement. Nothing is decided based solely on an estimate.
01
Environmental Audit
We’re taking stock of what actually exists: mailboxes and volumes, archives, public records, delegations, domains, identities, and terminals. And above all, application dependencies —the ones no one documents and that everyone discovers too late.
This is the step that most projects skip, and it’s the one that determines everything else. Nothing can move forward until it’s done.
What We Deliver
- Quantitative inventory: boxes, volumes, archives, public records, delegations
- The Dependency Map: Applications, Scanners, SMTP Relays, Connectors
- An end-to-endassessment of domains and email traffic
- The identified risks, along with how much each would cost if it occurred
02
Defining the Strategy
We determine the method for migration, the tools, the timeline, the user groups, and the migration plan. First, the approach: Microsoft 365, Exchange Server SE, or hybrid.
Everything is documented and discussed before we even touch the first server, including the options we ruled out and why. An architectural decision whose rationale is unknown is rehashed every time there’s a personnel change.
What We Deliver
- The chosen path, and the reasons for ruling out the others
- The wave-based breakdown: services, sites, subsidiaries, or areas
- The schedule, including the work windows negotiated with the various trades
- The contingency plan, written before the first shift and not during it
03
Preparing for the Trip
We are setting up Microsoft 365 or the new infrastructure Exchange : licenses, domains, security, email flows, and hybrid configuration when necessary.
The destination is hardened before it is populated. A mailbox migrated to a misconfigured tenant must be restored, and restoration is more expensive than the initial configuration.
What We Deliver
- The host or servers that have been set up, secured, and documented
- The proven end-to-end messaging flow, both incoming and outgoing
- The Hybrid Configuration When Coexistence Is Necessary
- Security and Compliance Rules: Retention, Logging, Protection
04
Pre-migration data
Data is transferred before the switchover whenever the method allows it. Successive synchronization passes reduce the remaining volume of data to be moved, and thus the disruption experienced by users.
We measure the actual throughput instead of estimating it: it is the throughput—not the number of mailboxes—that determines the duration of a switchover. If the measurement contradicts the schedule, the schedule is adjusted.
What We Deliver
- First, the complete pass; then, the differential synchronizations
- The actual measured flow rate, and the adjusted schedule, if necessary
- A pilot group that was selected, used, and validated before all the others
- The Report on Discrepancies Between the Source and the Destination
05
Submit and Confirm
We're finalizing the synchronization, activating the new environment, and then checking what almost no one else checks: delegations, shared folders, applications, scanners, and the flow of messages in both directions.
The first few days are all about the users, not the console. We'll remain available during this time, and the engineers who built the web migration will be the ones responding.
What We Deliver
- Switching, either in successive waves or all at once
- The checklist reviewed item by item and signed
- Supporting Users During Their First Few Days
- Dismantling the old environment, whenever you decide to do so
The migration can be organized in successive waves to spread out the workload and maintain temporary coexistence between the two environments, or through a coordinated switchover when the size of the organization allows it. The choice is made during the scoping phase, based on your operational constraints—not our standard practices.
Let's talk about your email
What is your situation?
Seven reasons to open this file. If any of them sound familiar, you've come to the right place—and we've seen this happen before.
Your infrastructure is reaching the end of its support period
You need to chart a course toward a secure, well-maintained environment, and you don't have much time left to do so calmly.
Your company is merging, splitting up, or restructuring
Users, domains, and data must be migrated to a new tenant without losing access or interrupting external communications.
Would you like to reduce your on-premises infrastructure?
Fewer servers to maintain, back up, and replace; more services entrusted to Microsoft 365.
Some applications still rely onExchange
ERP, scanners, alert systems, or line-of-business applications must continue to send messages after the migration. This is the leading cause of post-switchover incidents.
Your requirements call for a specific architecture
Data localization, compliance, security, or application constraints may require an on-premises or hybrid environment. That’s whereExchange Server SE comes into play.
Your email must remain available
The shift schedule is organized around your hours, your users, and the actual urgency of your communications—nights, weekends, and time slots by department.
Are you looking for a team to lead the migration
You want to maintain control over your environment while entrusting the preparation and execution of the project to specialists who have done this before.
Why Choose Us
Complex environments that we've encountered before.
Lambert Consulting Microsoft Solutions Partner —Modern Work— and supports Swiss and international organizations in their projects Exchange and Microsoft 365 since the product’s earliest versions.
Our services can cover the entire project or complement the expertise of your in-house team: auditing, architecture, preparation, migration, validation, and knowledge transfer. We don’t intend to stay; our goal is to ensure that your administrators know how to maintain the environment after we’re gone.
5 TB
Migration From one instance to the next: 500 users, their mailboxes, and their archives
1,400
Users migrated to Microsoft 365 within the same program
2,700
Mailboxes consolidated into a single environment Exchange
What's included
- A single point of contact for email, the directory, and workstations. The three go hand in hand; it makes sense for a single company to manage them all.
- Thirty Years of Microsoft Projects in French-speaking Switzerland, from the Historic Mail Server to the Tenant Microsoft 365.
- On-site teams: Renens, Sion, Châtel-Saint-Denis. No one opens a support ticket from the other side of the world on the night of the switchover.
- Truly, thebest of both worlds: we’re Exchange Online and are still operating Exchange Server. So we have no reason to favor one over the other.
- Federal authorization for staffing services since 2018: We can also assign an engineer to your site for the duration of the project.
Frequently Asked Questions
What we're asked to do before we begin.
The six questions that come up on every first date. The answers are intentionally specific: a vague answer at this stage doesn't help anyone.
Can a migration Exchange can it be carried out without interruption?
Most of the data can generally be transferred before the switchover, while everyone is working as usual. However, a short transition period is still necessary to complete the synchronization, modify the email flow, and connect users to the new environment.
This phase can be planned: one night, one weekend, or shift by shift. What doesn’t exist is a migration where nothing happens; what does exist is a migration where the interruption is brief, announced in advance, and comes as no surprise.
How long does a migration ?
The duration depends on the number of mailboxes, the volume of data, the available bandwidth, the archives, and technical dependencies. An environment with 100 users and no specific dependencies is nothing like an environment with 1,000 users connected to a ERP and 40 scanners.
The initial audit makes it possible to establish a realistic schedule and, most importantly, to measure the actual throughput rather than estimate it. It is the throughput—not the number of boxes—that determines the duration.
What data can be transferred?
Depending on the initial environment and the tools selected, the migration may include emails, calendars, contacts, archives, shared mailboxes, delegations, rules, and public folders.
Some items require a decision rather than a transfer: public files, for example, have no direct equivalent in Microsoft 365. We’ll walk you through the options—shared folder, group, site SharePoint — and you decide together with the relevant business units.
Is it possible to migrate only some of the users?
Yes. The migration can be organized by groups, departments, subsidiaries, or business units in order to phase in the transition and maintain a temporary coexistence between the two environments.
In fact, this is the most common approach in organizations with more than 200 employees: you start with a pilot group that agrees to report anomalies, and then you move forward in waves.
Do you have to go to the cloud ?
No. Exchange Server The Subscription Edition allows you to keep your email system on-premises, running a supported version with modern lifecycle management. It’s the right solution when data localization, regulatory requirements, an isolated network, or a line-of-business application make it necessary.
What’s no longer possible is staying on Exchange 2016 or 2019 and hope they’ll hold up: these versions no longer receive security patches, and the Extended Support program ends at the end of October 2026.
What happens to the last server Exchange after a migration full reset?
An organization that synchronizes its accounts from a local directory generally maintains a server Exchange — or at least its management tools — to administer the email attributes of the directory entries.
This server must also be running a supported version. That is why Exchange Server SE also applies to organizations that have already migrated to Microsoft 365, and this is something many people discover only after the fact.
All About Email
That's what a migration Exchange always touches.
A messaging service doesn't stand alone. Here are the other building blocks we're putting in place—and that projects Exchange almost always end up interacting with.
Let's talk about it
Let's prepare your migration Exchange.
A migration successful projectbegins with a thorough understanding of the existing environment, the data to be migrated, and the services that rely on messaging. The proposal is free of charge: it coversthe scope, workload, timeline, and deliverables, with no obligation on your part. The audit, however, is the first step of the project—that’s where the work begins.