How to migrate your business to Microsoft 365 without email downtime
An email migration is a change every employee notices the moment it stops working. With proper preparation, you can make the switch so smooth that almost nobody notices.
Contents
Preparation: what to establish first
Most problems don't arise during the switch itself, but because someone dived in without an inventory. Before you enable anything, you need a clear picture of what you're moving.
- Number of mailboxes and their size — this drives both migration time and licensing.
- Source system — on-premises Exchange, IMAP, Google Workspace or another provider.
- Shared mailboxes, distribution lists, calendars and contacts.
- Devices and apps that send mail over SMTP (printers, ERP, monitoring).
- Where the domain is registered and who has access to its DNS.
Check mailbox sizes for real. "We only have a few emails" often turns out to be a 40 GB archive that migrates overnight.
Choosing the migration type
Cutover
All mailboxes at once during a single window. Ideal up to roughly 150 mailboxes — the most common choice.
Staged
Mailboxes in batches. Makes sense with larger counts or a slower connection.
Hybrid
On-premises Exchange and Exchange Online run side by side and hand mail off seamlessly. More complex, but essential where zero interruption and full calendar coexistence are required.
If you can't clearly justify why you need hybrid, cutover almost certainly does the job.
DNS and the domain
A single record — MX — routes your mail. The key is to prepare everything in advance and lower the TTL so the change propagates quickly.
- A day or two before, lower the MX TTL to 300 seconds.
- Verify domain ownership in Microsoft 365 (a temporary TXT record).
- Prepare final MX, SPF and autodiscover values — don't apply them yet.
After switching, don't forget SPF, DKIM and DMARC — without them your new mail may land in spam.
The cutover
- Start the migration batch and let it sync content in the background.
- Once synced, switch the MX record to Microsoft 365.
- Configure autodiscover so Outlook and mobiles switch automatically.
- Reconfigure devices that send mail over SMTP.
Don't shut down the old server immediately. Leave it running for a few days — mail delivered before the MX change propagates may still reach it.
Post-migration checks
- Send and receive a test from both external and internal addresses.
- Verify shared mailboxes, calendars and distribution lists.
- Check signatures, rules and automatic replies.
- Set up backup — Microsoft 365 is not a backup.
Common mistakes
- Undiscovered shared mailboxes that suddenly deliver to nobody.
- Forgotten autodiscover — profiles have to be set up by hand.
- High TTL, so part of the world still sends to the old server for hours.
- Assuming cloud means backup.
Planning a move to Microsoft 365?
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