How to Keep Mail at a Third-Party Host During Microsoft 365 Migration
Question details
The user needs incoming emails, including those originating from other Microsoft 365 tenants, to continue arriving at their existing third-party mail provider while their domain is being migrated to Microsoft 365.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- A domain has been verified in Microsoft 365, but the email migration is still in progress, requiring the mail flow to remain on the old third-party host.
- Observed behavior
- The goal is to maintain uninterrupted mail delivery to the third-party provider and avoid routing mail prematurely to the new Microsoft 365 environment before the migration completes.
Ensure you have administrator access to your domain's DNS hosting provider and verify your current MX record settings before proceeding with the domain verification in Microsoft 365.
Delay Updating the MX Record Until Migration is Complete
Keep your domain's MX record pointing to your third-party mail host to ensure all incoming emails are routed there during the transition phase.
Email routing relies on your domain's MX (Mail Exchanger) records. Even when your domain is verified in Microsoft 365, mail originating from external sources or other Microsoft 365 tenants will perform a DNS lookup.
As long as your MX record still points to your third-party provider, Microsoft 365 will treat emails directed to your domain as external mail and route them accordingly to your existing host.
In the Microsoft 365 admin center, verify your domain using a TXT record. Do not select the option to have Microsoft 365 manage your DNS records automatically.
When prompted to add DNS records for email (such as MX, SPF, and CNAME), skip this step or choose to do it later. Leave your existing MX record untouched at your DNS host.
Migrate your users' mailbox data to Microsoft 365 using your preferred migration method while they continue using the third-party provider for daily communication.
Once all data is migrated and users are ready to switch over to Microsoft 365, log into your DNS host and update the MX record to point to Microsoft 365 to finalize the cutover.
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Frequently Asked Questions
Can I verify my domain in Microsoft 365 without affecting current email delivery?
Yes. You can verify domain ownership using a simple TXT record in your DNS settings. As long as you do not update the MX record, your existing email delivery flow will remain completely unaffected.
Why do emails from other Microsoft 365 tenants still go to my third-party host?
Microsoft 365 respects standard DNS resolution protocols. If your domain's MX record points to a third-party host, Microsoft 365 treats the message as external mail and routes it there, regardless of whether the sender is using Microsoft 365.
When is the correct time to change the MX record to Microsoft 365?
You should only update your MX record to point to Microsoft 365 after all user mailboxes have been successfully migrated and your users are fully configured and ready to log in to their new Microsoft 365 accounts.
What happens if I change the MX record before the migration is finished?
If you update the MX record prematurely, new emails will start delivering to the empty or partially migrated Microsoft 365 mailboxes. This creates a split-brain scenario where users must check two different inboxes to see all their mail.
