Microsoft 365 Migration Checklist — 27 Steps Before, During & After (2026)

Count everything: user mailboxes, shared mailboxes, room/equipment mailboxes, distribution groups, mail-enabled security groups, and mail contacts. Export the list with sizes. Do not guess — run Get-Mailbox -ResultSize Unlimited | Select Name,PrimarySmtpAddress,TotalItemSize in Exchange Management Shell.

If you have public folders, document them now. How many? How large? Who uses them? Public folders are the most common source of migration delays. Run Get-PublicFolder -Recurse | Get-PublicFolderStatistics to get the full picture.

Screenshot or export every DNS record for your email domain: MX, autodiscover, SPF, DKIM, DMARC, any CNAME records. You will need to change some and you want to be able to roll back if something goes wrong.

Can you actually log into your DNS provider and make changes? Who has the credentials? I have seen migrations delayed 3+ days because nobody could find the GoDaddy login. Verify access NOW, not at 11pm on cutover night.

Every user being migrated needs a license with Exchange Online. Business Basic, Business Standard, E3, E5 — whatever fits. Buy them, assign them, and make sure they are active before you start migrating. Unlicensed mailboxes will not receive migrated data.

Go to admin.microsoft.com → Settings → Domains. Add your domain and complete TXT record verification. Do NOT change MX records yet — that comes during cutover.

Full backup of your Exchange databases. Backup of your Active Directory. Backup of your current DNS. If you use any third-party tools (backup software, archive solutions), make sure those are current too. This is your safety net.

For hybrid or directory sync scenarios, install and configure Entra Connect. This syncs your on-prem AD users to Microsoft 365. Allow 24–48 hours for initial sync and troubleshooting.

Set up migration endpoints in Exchange Online — this tells M365 how to connect to your on-prem Exchange. You need Outlook Anywhere (EWS) enabled, a proper SSL certificate (not self-signed), and firewall rules allowing Microsoft's IP ranges.

Pick 5–10 users for the first batch. Choose a mix: IT staff, a power user, someone who barely uses email, and someone with a large mailbox. This group validates the process before you migrate everyone.

Write three emails: (1) "Migration is coming — here is what to expect" sent 1 week before, (2) "Migration starts tonight/this weekend" sent day-of, (3) "Migration complete — here is what to do" sent after cutover. Clear, simple, no jargon.

Have users empty Deleted Items, archive old mail, and remove unnecessary attachments. Every GB you do not migrate saves time and reduces risk. Set a deadline: "Clean up by Friday or it all comes with you."

Move your pilot group and wait 24 hours. Verify their mail flow, calendar, contacts, and any shared resources work correctly in the new environment. Do not proceed until the pilot is clean.

Group remaining users into batches of 50–100 (or whatever your bandwidth supports). Stagger start times. Monitor each batch for errors before starting the next one. Do not just fire off 500 mailboxes at once.

Check the migration dashboard in Exchange Admin Center every few hours. Look for failed items, stalled mailboxes, and throttling. Common issues: large items exceeding the 150 MB message limit, corrupted items, and mailboxes that have been moved or renamed since the batch was created.

You will get errors. Some mailboxes will have items that cannot migrate (corrupted messages, oversized items). Document these, skip them if needed, and deal with them post-migration. Do not let one problem mailbox hold up the entire batch.

Before DNS cutover, run a final delta sync to catch any new mail that arrived since the initial migration. This minimizes the gap between old and new environments.

This is the moment of truth. Change your MX record to point to Microsoft 365 (the exact value is in your M365 admin portal). Update autodiscover CNAME. Update SPF to include include:spf.protection.outlook.com. TTL should have been lowered to 300 seconds 24–48 hours before this step.

Send test emails: internal-to-internal, external-to-internal, internal-to-external. Check that replies work. Verify that mail is hitting the new M365 mailboxes, not the old server. Do this from multiple devices and email clients.

Users will likely need new Outlook profiles or at minimum need to restart Outlook and let autodiscover reconfigure. In some cases, you will need to create new profiles manually. Have instructions ready for both Windows and Mac. New Outlook for Windows handles this automatically; classic Outlook often does not.

iPhones and Android devices need updated Exchange settings. Most will auto-detect via autodiscover, but some will need manual reconfiguration. ActiveSync devices are especially finicky. Send users step-by-step instructions or schedule a walk-in support window.

Shared mailboxes should be accessible. Distribution groups should be delivering. Test every shared mailbox — do not assume. Permissions often need to be reassigned in Exchange Online.

Enable DKIM for your domain in Microsoft 365. This improves email deliverability and is increasingly required by receiving mail servers. Go to security.microsoft.com → Email authentication settings → DKIM. Create and publish the CNAME records, then enable signing.

If you had a DMARC record (you should), make sure it still works with the new mail flow. If you did not have one, now is the time. Start with v=DMARC1; p=none; rua=mailto:[email protected] and tighten it over time.

Watch for bounce-backs, delivery delays, missing emails, and user complaints. Check message traces in the Exchange Admin Center for any anomalies. The first two weeks post-migration are when problems surface.

Do not just unplug it. Properly uninstall Exchange, remove AD objects, clean up DNS entries that still point to the old server, and securely wipe the hardware (or terminate the VM). Keep a final backup for 90 days just in case.

Write up what was migrated, what was skipped, what issues were encountered, and how they were resolved. Future-you will thank present-you when someone asks "why does this mailbox only have mail from 2020 forward?" six months from now.