If you are planning an email migration to Microsoft 365 for your small business, the two things most likely to go wrong are losing mail already sitting in your old mailboxes and missing incoming messages during the switchover. Both problems are preventable, and you do not need a large IT team to avoid them.
This post is for owners and office managers at organizations with fewer than 150 staff who are moving from an on-premises Exchange server, a hosted email service, or an IMAP-based provider to Microsoft 365. The decisions covered here matter most in the days before and after your cutover.
Email Migration to Microsoft 365: Choosing Your Method
Microsoft 365 supports several migration paths. The right one depends on where your mail currently lives and how many mailboxes you are moving.
- Cutover migration moves all mailboxes at once. Microsoft recommends cutover for 150 mailboxes or fewer, even though the technical ceiling is 2,000. Above 150, the initial sync takes long enough that controlling the migration window becomes difficult.
- Staged migration moves mailboxes in batches. Exchange 2010 and 2013 are not supported for staged migration and must use cutover or hybrid migration instead.
- Hybrid migration suits larger or more complex environments that need on-premises Exchange and Exchange Online running side by side for an extended period.
- IMAP migration works when you are moving from a non-Exchange system such as Gmail or a shared hosting provider. The important limitation: IMAP migration moves only email messages; contacts, calendar items, and tasks are not migrated and must be moved separately.
Pre-Staging Mailboxes: What It Means and Why It Prevents Data Loss
Pre-staging means running the initial mailbox sync before you flip the MX record. Your old mail server keeps accepting inbound mail during this phase while the migration engine copies existing messages and folders into Exchange Online. When the MX record finally changes, users' mailboxes already have their full history in place.
This is the step most often skipped when migrations go wrong. Mail sent to on-premises mailboxes is routed to the on-premises Exchange server until the MX record is changed, not when the mailboxes are moved. Flip the MX record before the initial sync finishes and new messages arrive in Exchange Online while old messages are still being copied, which can leave them in separate places or cause bounces.
For staged migrations, Exchange Online handles forwarding automatically. The TargetAddress property on each on-premises mailbox is configured with the email address of the Exchange Online mailbox, so messages sent to the old address are forwarded to the new one while the migration is in progress. Staged migration also has one hard prerequisite: Microsoft Entra ID Sync must be running between your on-premises Active Directory and Microsoft 365 or the migration batch will fail before a single mailbox moves.
The MX Record Change: When to Do It and How Long It Takes
The MX record (mail exchanger record) tells the internet where to deliver email for your domain. Changing it is the moment mail starts flowing to Microsoft 365 instead of your old server. How quickly that change spreads across the internet depends on the TTL (time-to-live) setting at your domain registrar.
Lower your MX record's TTL to 5 minutes well before your planned cutover. Then schedule the actual MX change for your lowest-traffic window, typically a Friday evening or early Saturday morning. After you flip the record, leave the old mail server accepting messages through the propagation window. Mail in transit from servers that have not yet picked up the new MX record will still arrive at your old server, and you need somewhere for it to land.
If you are using IMAP migration, there is an important timing issue: once an IMAP migration is complete, any new mail sent to the source email system is not automatically migrated to Microsoft 365. Messages that arrive at your old server after the sync ends stay there. Switch the MX record promptly and keep the old system accessible until you have confirmed all mail is landing in Exchange Online.
What to Tell Your Team Before and After Cutover
Users do not need to understand the technical details, but they need a few specific instructions. Send a note to your team well before the maintenance window so they have time to prepare.
- Tell them when the window starts and roughly how long to expect disruption.
- Ask them to save any drafts and close Outlook before the window begins.
- Let them know they will need to re-enter their credentials in Outlook and on their phones and tablets after the migration.
- Anyone with Out of Office replies configured will need to recreate those in Microsoft 365. Existing Out of Office settings are not carried over from the old system.
One step that is easy to miss: licenses must be assigned before users can access their new mailboxes. Cutover migration creates Microsoft 365 accounts automatically, but the license assignment is a manual step in the admin center. Complete it before the MX flip or your team will be locked out the moment mail arrives.
Step-by-Step Migration Checklist
- Inventory all mailboxes, including shared mailboxes, distribution lists, and resource mailboxes. These must be created in Exchange Online manually before cutover.
- Assign Microsoft 365 licenses to all users in the admin center.
- Create a migration endpoint in the Exchange Admin Center (EAC) using your on-premises Exchange server credentials.
- Create a migration batch and start the initial sync. Let it run to completion. Initial synchronization processes 20 mailboxes at a time by default, so larger batches take longer.
- Lower your domain's MX record TTL to 5 minutes well before the cutover window.
- At the scheduled time, update the MX record to point to Microsoft 365.
- Update the Autodiscover DNS record so Outlook and mobile clients can locate the new server automatically.
- Confirm mail is flowing into Exchange Online before sending the all-clear to your team.
- Leave the old mail server running and accepting messages through the DNS propagation period after the MX change.
- Once propagation is complete, check the old server for any messages that arrived during that window, then shut it down.
Common Mistakes and How to Avoid Them
A common mistake is treating shared mailboxes and distribution lists as an afterthought. They are not created automatically during a cutover migration. If your team uses a shared "info@" or "support@" inbox, create it in Exchange Online with the correct permissions before the MX record changes.
Mobile devices are another frequent blind spot. Every phone and tablet with the old mail account configured will need to be reconfigured after cutover. Plan for this during the maintenance window, not the following morning when your team is trying to start the day.
After the migration, update your IT documentation to reflect the new DNS settings, mailbox inventory, and license assignments. The IT Documentation Checklist Every Small Business Needs gives you a practical starting point. If this migration is part of a plan to retire aging on-premises hardware, The Real Cost of Keeping a Server in Your Office Closet walks through when decommissioning makes financial sense.
JDL manages email migrations and ongoing Microsoft 365 support as part of our Managed IT Services for small and mid-sized organizations. If you are also evaluating a broader move to cloud-hosted infrastructure, our Cloud and Infrastructure page covers what that typically involves.
If you would like to walk through your specific setup before choosing a migration method, book a free 30-minute call with Joshua. Describe your current environment and you will leave with a clear next step and no sales pressure.
