The fear around email migration is that everything stops for a day. It is a reasonable fear and, done properly, an unfounded one. Mail is one of the few systems that can be moved with essentially no visible interruption, because the data can be copied while the old system keeps running.
Why there is almost no downtime
A migration is two separate things that people conflate. Copying the data takes days or weeks and happens entirely in the background with the old system still live. Switching the delivery route takes minutes. Only the second one has any user impact at all.
Cutover day, hour by hour
| When | What happens | What staff notice |
|---|---|---|
| Days before | Historic mail syncs to the new platform | Nothing |
| Days before | MX record TTL lowered so the change propagates fast | Nothing |
| Evening, hour 0 | Final delta sync runs | Nothing |
| Hour 0 | MX record changed to the new platform | Nothing |
| Hours 0 to 2 | New mail begins arriving at the new platform | Nothing, they are not working |
| Next morning | Staff open a reconfigured profile | One-time sign-in, mail is all there |
| First day | Old mailbox still receives stragglers, synced across | Nothing |
What actually goes wrong when it does
- Shared mailboxes and aliases nobody inventoried, discovered when someone reports a missing inbox
- TTL left high, so DNS propagation takes hours instead of minutes and mail splits between platforms
- SPF, DKIM and DMARC not updated before cutover, so outbound mail lands in recipients' spam folders
- Mobile devices never reconfigured, so staff assume mail has stopped when their phone is simply pointing at the old server
- Mailbox size limits hit mid-sync, which stalls a migration quietly if nobody is watching
- The old system decommissioned too early, removing the rollback option and any straggler mail
The spam-folder problem is DNS
Deliverability problems after a migration nearly always trace back to authentication records that were never updated. It is not a mail server problem and no amount of restarting fixes it.
What we do to keep it boring
- 1Inventory shared mailboxes, aliases, distribution groups and forwarding rules before anything moves
- 2Pre-sync all historic mail while the current system stays fully live
- 3Lower MX TTL several days ahead so the switch propagates in minutes
- 4Configure SPF, DKIM and DMARC before the cutover, not after the complaints
- 5Cut over outside business hours and verify mail flow end to end before anyone starts work
- 6Keep the old mailboxes live and receiving for an agreed period, so rollback stays available
The one thing staff should be told
Send a short note beforehand explaining that they will sign in once, that their mail will all be there, and who to contact if anything looks wrong. Most migration complaints are not about mail failing. They are about people not knowing what to expect.
Planning a mail migration?
We audit the current setup first, including the shared mailboxes nobody remembers, then pre-sync and cut over out of hours with a written rollback plan.
Frequently asked questions
How long will our email be down?
For most businesses, not at all in any way staff notice. Historic mail copies across while the old system stays live, and the cutover itself is a DNS change made outside business hours. Users sign in once the next morning and their mail is there.
Will we lose any email during the switch?
No, if the old mailboxes are kept live and receiving through the cutover. Mail arriving at the old platform during DNS propagation is delivered normally and synced across. Problems occur when the old system is decommissioned too early.
Why did our mail start going to spam after migrating?
Almost certainly because SPF, DKIM and DMARC records were not updated to reflect the new sending platform. It is a DNS configuration problem rather than a mail server one, and it should be fixed before the cutover rather than after.
What about mail on our phones?
Mobile profiles need reconfiguring to point at the new platform. If this is not communicated, staff see mail stop arriving on their phone and conclude the migration failed, when in fact their device is still asking the old server. Plan the instructions in advance.
Can we go back if something goes wrong?
Yes, provided the old system stays live and reachable, which we insist on for an agreed period. Rolling back is a matter of changing the MX record back. It is rarely needed, but a migration without that option is a gamble rather than a plan.
Usman K.
· IT Support LeadIT support lead at Azizi Technologies. Manages 24/7 helpdesk, Microsoft 365 migrations, server administration, and managed IT contracts for Dubai SMBs. Microsoft Certified. Mentioned by name in client reviews for fast resolution.
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