Exchange Server to Exchange Online migration checklist

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Part 1

Choose the method

Count mailboxes and total sizeThe count decides which methods are practical.
Consider cutover for small organisationsCutover supports up to 2,000 mailboxes, but Microsoft recommends 150 or fewer.
Consider hybrid for larger or phased movesHybrid gives the look and feel of a single Exchange organisation during the move.
Run the mail migration advisorMicrosoft's advisor walks you to the right method for your source.
Check your Microsoft 365 plan supports hybridMicrosoft 365 Apps for business and Home plans do not support hybrid deployments.

Part 2

Install the latest CU on ExchangeHybrid needs the latest Cumulative Update, or the one immediately before it.
Deploy Microsoft Entra Connect or cloud syncDirectory synchronisation is required for a hybrid deployment.
Get a certificate from a trusted public CASelf-signed certificates are not supported. Include the EWS external URL and Autodiscover in the SAN.
Point Autodiscover public DNS to on-premisesDuring hybrid, Autodiscover for your SMTP domains points to your on-premises Exchange servers.
Open TCP 25 and 443Port 25 carries hybrid mail flow. Port 443 carries Autodiscover, EWS and mailbox moves through MRSProxy.
Add your accepted domains to Microsoft 365At least one on-premises accepted domain must be added before the wizard continues.
Run the Remote Connectivity AnalyzerMicrosoft strongly recommends this check before you run the Hybrid Configuration Wizard.

Part 3

Run the wizard from a supported serverUse Exchange 2016 CU8 or later, or Exchange 2013 CU15 or later, on a domain-joined computer.
Sign in with a Global Administrator accountThe wizard needs Global Administrator rights in Microsoft 365.
Choose Full Hybrid ConfigurationSelect the transport certificate and enter the external FQDN, such as mail.contoso.com.
Verify the configurationRun Get-HybridConfiguration and read the Update-HybridConfiguration log on the Mailbox server.
Test a mailbox move and free/busyMove a test mailbox and check calendar sharing with a cloud mailbox.

Part 4

Verify your domain in Microsoft 365The on-premises domain must be a verified domain before a cutover migration.
Configure Outlook Anywhere with a trusted certificateThe migration service connects through Outlook Anywhere. Enable MRSProxy too.
Grant the migration account accessIt needs FullAccess on each mailbox or Receive As on the mailbox database.
Unhide mailboxes from address listsMicrosoft lists hidden mailboxes as a cutover blocker.

Part 5

Start with a pilot batchWatch source server performance during the pilot.
Set a sensible concurrencyThe default is 20 concurrent moves, up to 100. Microsoft suggests starting low, for example 10, on a weak source.
Schedule moves out of hoursAvoid running backups or Exchange store maintenance during migration.
Allow Microsoft 365 IP ranges through firewallsCheck intrusion detection is not slowing migration traffic.
Monitor batches and fix failuresUse Get-MoveRequestStatistics to read the status detail for stuck users.

Part 6

Lower the MX TTL ahead of timeMicrosoft suggests 3,600 seconds or less before the change.
Point MX at Exchange OnlineChange MX once mailboxes are migrated. See the DNS and email cutover guide.
Wait at least 72 hours before deleting a cutover batchCheck Last Synced Time is later than when mail started routing to Microsoft 365.
Assign licences to migrated usersWithout a licence the mailbox is disabled when the 30-day grace period ends.
Create the Autodiscover CNAMEAlias autodiscover with target autodiscover.outlook.com.
Decommission Exchange only after mail routes directly to Microsoft 365Microsoft recommends contacting support first because decommissioning can have unintended consequences.

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