Microsoft 365 Migration Checklist for UK Businesses

A Microsoft 365 migration is not just moving mailboxes. Files, calendars, shared inboxes, mobile devices and security policies all need a plan — and the order matters. Get one step out of sequence and you can end up with users receiving mail in two places for weeks.

Before you start

• List every mailbox, shared mailbox and distribution group in scope.
• Inventory shared file locations, sizes and their owners.
• Note calendar-sharing and delegation set-ups so nothing is lost in translation.
• Identify any line-of-business systems that send email — accounts, CRM, scanners, alarms.
• Confirm DNS access for MX, SPF, DKIM and DMARC changes on the day.
• Decide on licensing — Business Basic, Standard, Premium, or a mix.

The migration itself

• Provision users and licences a week ahead. Do not leave this to cutover day.
• Run a first-pass mailbox sync while the old system stays authoritative.
• On cutover, run a final delta sync and then flip MX records.
• Reconfigure send-connectors and any third-party mail relays.
• Roll out Outlook re-profiling to each device.
• Migrate OneDrive / SharePoint content in parallel — usually the longest task.

Security you should turn on from day one

• Multi-factor authentication for every user — no exceptions.
• Conditional access to block unusual sign-in locations.
• DKIM signing for outbound mail.
• DMARC policy set at least to “quarantine”.
• Retention policies and legal hold if required by your industry.

Common pitfalls to avoid

• Migrating users before licences are actually assigned — mailboxes fail to provision.
• Not updating internal DNS records for services that rely on the old mail domain.
• Missing shared mailboxes — they need explicit assignment.
• Forgetting mobile-device Exchange profiles — users think “email is broken” on day one.