Moving to Microsoft 365 without e-mail downtime

Whether it comes from an in-house Exchange server, an e-mail hosting provider, Google Workspace or a packaged offering, a migration to Microsoft 365 always follows the same logic: prepare, copy in the background, switch over at a quiet time, then check.

Done properly, it involves no lost e-mail and only limited disruption for your teams.

Preparing the migration

• Inventory mailboxes, aliases, shared mailboxes, distribution lists and shared calendars.
• Measure the volumes to migrate, which determine how long the copy takes.
• Identify who manages the domain name and the DNS zone, and obtain access.
• Choose the right licenses for each user profile.
• Delete inactive accounts rather than migrating them.
• Announce the cutover date and what will change for users.

Cutover steps

• Create the tenant, verify the domain and create the accounts with multi-factor authentication.
• Copy mailbox contents in the background, while the old system is still running.
• Change the DNS records (MX, Autodiscover, SPF) at a time of low activity.
• Run a final synchronization to pick up messages that arrived in the meantime.
• Reconfigure Outlook and phones, then check sending and receiving.
• Enable DKIM and DMARC for the domain.

What about files?

Files do not have to follow the same schedule. Depending on how they are used, they can move to SharePoint and OneDrive, or stay on a file server. A file migration means reviewing access rights and folder structure, and checking for paths that are too long or incompatible file names.

Classic pitfalls

• Printers and scanners that send e-mail and need reconfiguring.
• Business applications, websites and invoicing tools that send messages on behalf of the domain.
• Automatic forwarding and generic addresses left out of the inventory.
• Removing the old system before the end of the verification period.

After the cutover

• Extra support in the first few days, for practical questions.
• An overlap period during which the old system remains accessible.
• Setting up a dedicated backup for Microsoft 365.
• Updating the documentation: accounts, licenses, DNS, settings.