Microsoft 365 Migration
Checklist

Before you start — discovery checklist

Work through these items before any migration planning begins.

Environment inventory

• Count all mailboxes, including shared mailboxes and resource rooms
• Document all distribution lists and mail-enabled groups
• List all email domains in use
• Identify all mail connectors and custom mail flow rules
• Document any email aliases on each account

Data assessment

• Measure total mailbox size per user and in aggregate
• Identify users with very large mailboxes (over 20GB) for special handling
• List all SharePoint sites and their owners
• Measure total OneDrive usage across the organisation
• Identify any Teams channels with significant file content

Third-party integrations

• List every application that connects to your email (CRM, accounting, helpdesk, etc.)
• Identify which applications use OAuth or application passwords
• Note any applications that send email via SMTP relay
• Check whether any applications are hardcoded to specific email server addresses

DNS and domain

• Confirm you have access to manage your domain's DNS records
• Document current MX record values
• Document current SPF, DKIM, and DMARC records
• Identify who controls DNS (domain registrar, hosting provider, IT team)

SPF, DKIM, and DMARC deserve particular attention here — getting these records wrong is one of the most common causes of post-migration deliverability problems. See our guide to SPF, DKIM, and DMARC after a Microsoft 365 migration for the full configuration sequence.

Pre-migration checklist

Complete these before migration begins.

Licensing and accounts

• Procure Microsoft 365 licences for all users in the destination tenant
• Create all user accounts in the destination tenant
• Assign licences to all accounts
• Configure any shared mailboxes and resource rooms

Security baseline

• Enable multi-factor authentication (MFA) for all accounts before cutover
• Configure conditional access policies if required
• Set up Microsoft Defender for Business or equivalent
• Review and configure anti-spam and anti-phishing policies

Communication

• Inform all users of the migration date and expected changes
• Prepare user guidance for reconnecting their email client post-cutover
• Identify a point of contact for user support on migration day
• Set expectations for the support period after cutover

Rollback plan

• Confirm source environment will remain intact during and after migration
• Document rollback steps in case cutover needs to be reversed
• Confirm decision-maker availability on migration day for go/no-go decisions

Cutover day checklist

These should happen on the day of migration in order.

• Confirm all pre-staging is complete and data is synced
• Run final delta sync to capture recent changes
• Update MX records to point to Microsoft 365
• Update SPF record to include Microsoft 365 sending addresses
• Update DKIM configuration in Microsoft 365 admin
• Verify email is flowing into new tenant
• Send test emails and confirm delivery
• Update Autodiscover DNS record
• Begin user communication that cutover is complete
• Open support channel for immediate user issues

Post-migration checklist

Complete these in the days following cutover.

Integrity verification

• Confirm all mailboxes have migrated completely
• Spot-check email history for a sample of users
• Verify calendar events and recurring meetings are intact
• Confirm all contacts migrated correctly
• Check that shared mailboxes are accessible

Applications and integrations

• Reconnect all third-party applications to the new tenant
• Update any SMTP relay configurations
• Test all application email sending
• Confirm CRM and other integrations are syncing correctly

Cleanup

• Remove any migration permissions or delegated access used during migration
• Confirm DMARC is configured for the new tenant
• Archive or decommission the source environment after sign-off
• Update internal documentation to reflect the new environment

Common mistakes to avoid

Not inventorying third-party integrations. Applications that send or receive email often break silently after migration. Finding them before you start saves significant post-migration troubleshooting.

Underestimating data volumes. Large mailboxes and SharePoint sites take time to pre-stage. Underestimating volumes leads to longer-than-expected pre-staging and rushed cutovers.

Migrating without MFA enabled. Enabling MFA after migration is harder than enabling it before. Do it before you start.

No rollback plan. Every migration should have a documented rollback procedure. If something goes wrong during cutover, knowing exactly how to revert saves critical time.

CloudyNex builds every one of these steps into our Microsoft 365 migration service, backed by a documented, repeatable methodology. See our migration consulting service if you want an independent review of your plan before you commit.

This checklist covers the migration project itself. Once migration is complete, tenant security is a separate piece of work — see our guide to securing a Microsoft 365 tenant after migration for what comes next.