Before choosing the target
Migrations that fail are rarely technical failures. They are projects where management promised “the same thing, but European” when some of the uses could not follow. This first block sets what goes, what stays, and what the teams are told.
- Actual uses are listed: email, files, Teams, advanced Excel, business applications.
- What will not be migrated is written down and accepted by management.
- Employees are informed of what changes: webmail, online document editor, Teams history not transferred.
- The target’s online editor is tested on your own workbooks and documents, not on a demo.
- The target is chosen for these uses, not for a slogan. See the guide to alternatives.
- The quote separates subscription, migration and exclusions. See the cost.
- The quote states the volume assumed, the number of copy passes, the completion criterion and the price of a discrepancy discovered later.
- The operator’s commitment is in writing: arrival of messages, transfer of the history, or both.
- Internal time is planned: a named lead, available for the inventory, the sample and cutover day.
Inventory
The inventory determines the quote and the schedule. Every item forgotten here comes back later as rework, often at the worst moment. Applications that send mail are the most frequent gap: a copier or payroll software that keeps sending with the old credentials only shows up after the cutover.
- Domains and subdomains that send or receive mail.
- Named mailboxes, shared mailboxes, lists, aliases, forwards.
- Volume per mailbox and mailboxes under legal hold.
- Existing archives, and what must be retained.
- Accounts without an owner (employees who have left).
- Devices: Outlook for Windows, Outlook for Mac, mobiles, webmail only.
- Sending applications: copiers, website, CRM, alerts, payroll.
- Software that authenticates via SMTP, with the credentials it uses.
- Files in scope: OneDrive, useful SharePoint libraries, internal links and anonymous shares.
- Current DNS records: MX, SPF, DKIM, DMARC, autodiscover.
- End date of the current Microsoft subscriptions and termination conditions.
Preparing the target
The target is prepared while employees continue to work on Microsoft 365. The MX does not move. This phase is used to find problems on a sample, while there is still time to correct the method.
- Accounts created, passwords or invitations ready.
- Aliases and lists recreated.
- The target’s SPF, DKIM and DMARC ready to be published.
- First copy of the history started, MX unchanged.
- Sample checked: management, shared mailbox, large mailbox, empty mailbox.
- On the sample: number of messages, folders, contacts and recurring appointments verified.
- Files copied if the scope includes them, and opened on the target.
- One-page instruction sheet for Outlook and for mobile, with the “delete the account and recreate it” procedure.
- MX TTL lowered several days in advance.
- Written rollback plan: who restores the old MX, and within how many minutes.
The day before
The day before is used to reduce the gap between the two platforms to a minimum. Without a second pass, the last days of mail are missing. Without a freeze, an alias created that evening exists on only one side.
- Second copy pass (the delta).
- Freeze on alias creation and list changes.
- Internal hotline named for cutover day.
- Test send from the target to an external Gmail mailbox and an external Microsoft mailbox.
Cutover day
Cutover day is judged on external evidence, not on the fact that “it works internally”. A message sent from an outside personal mailbox must arrive on the new platform within the TTL; a real outgoing message must pass SPF, DKIM and DMARC at a recipient.
- MX switched.
- External test email received on the new platform.
- SPF, DKIM and DMARC validated on a real outgoing message.
- Copiers, website and CRM moved to the new SMTP.
- Sample of users reachable and able to send.
- Queue monitored throughout the day.
- The person authorised to restore the old MX is reachable all day.
Afterwards
The migration is not finished when the MX has been switched. It is finished when only one world sends mail, the discrepancies have been dealt with and a restore has been tested on the target. Shutting off sending on the old platform prevents an employee from replying from two places.
- Old platform read-only, no longer sending.
- Reported discrepancies dealt with (missing folder, delegation), targeted third pass if needed.
- Server-side rules, centralised signatures and delegation rights set up again.
- Mobiles reconfigured, old profiles deleted if they create duplicates.
- Closing date of the Microsoft tenant recorded, final export archived if necessary.
- The target’s retention rules and backups checked through a test restore.
- Record of processing activities and internal documentation updated with the new operator.
The step-by-step method is in migrating without downtime. The general order of operations and what does not transfer are in migrating from Microsoft 365 to a European solution.
Frequently asked questions
Who should own this checklist?
An internal lead, with an owner and a date on each line. The provider can carry out a large part of it, but validating the sample, the exclusions and the completion criterion is the company’s responsibility.
Can the sample be skipped if the copy tool is reliable?
No. The tool copies; it does not check that management’s shared mailbox or the recurring appointments have arrived correctly. The sample is the moment when the method is corrected at the lowest cost.
How long should Microsoft 365 be kept after the cutover?
Long enough to check that no flow still uses it and that the discrepancies have been dealt with, in read-only mode. Set the closing date in advance and record it, so as not to pay for two platforms for no reason.
Does this list apply to Google Workspace?
For the most part, yes: inventory, DNS, copy passes, sending applications, rollback. The points specific to Google (labels, Drive file owners, groups) are described in migrating from Google Workspace.
Klytic
For an email migration to Klytic: free advice, no email lost, migration carried out by Klytic on a quote basis. The subscription and the migration are two separate invoices; subscription prices are on the Pricing page.
This page describes a checklist. It does not replace the inventory of your tenant or the cutover plan written with your provider.
Sources
Accessed in October 2026.
