Route Mail to an External Server During a Microsoft 365 Domain Migration
Question details
A tenant administrator needs to route emails from one accepted domain to another in the same Microsoft 365 tenant using an external MX record rather than internal routing.
- Product
- Microsoft 365 / Exchange Online
- Device & OS
- not provided
- Scenario
- Migrating a domain from an old mail service to Microsoft 365, where some users still reside on the old external mail server.
- Observed behavior
- Exchange Online bypasses the external MX record and delivers messages internally because both domains are accepted in the same Microsoft 365 tenant.
Ensure you have Exchange Administrator or Global Administrator privileges in your Microsoft 365 tenant before modifying domain settings and mail flow connectors.
Change Accepted Domain Type and Create an Outbound Connector
Configure the migrating domain as an Internal Relay and set up an outbound connector to force Exchange Online to route unresolved mail to your old server.
When a domain is set to 'Authoritative', Microsoft 365 assumes it hosts all mailboxes for that domain and will drop messages for unknown users. By changing the domain type to 'Internal Relay', you instruct Exchange Online to forward messages to your old server if the mailbox does not exist in the cloud.
Log in to the Exchange admin center (EAC). Navigate to Mail flow > Accepted domains. Select your migrating domain, change its type to 'Internal Relay', and save the changes.
Ensure you do not have active Exchange Online mailboxes created for the users who still need to receive their mail on the old external service during the migration.
In the EAC, navigate to Mail flow > Connectors. Click the '+' icon to add a new connector. Choose 'Office 365' as the from location and 'Your organization's email server' as the to location.
Specify the domain that needs external routing. When prompted for how to route the messages, enter the IP address or the smart host (FQDN) of your old mail server.
Follow the rest of the wizard to validate the connection using an email address hosted on the old server, and save the connector once validation succeeds.
Verify Mail Flow via Message Trace
After modifying your domain and connectors, use the message trace tool to confirm that emails are properly routed externally.
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Frequently Asked Questions
Why does Microsoft 365 ignore my external MX record by default?
When a domain is added to a Microsoft 365 tenant, Exchange Online treats it as 'Authoritative' by default. This means it assumes it hosts all mailboxes for that domain and routes emails internally, intentionally bypassing external MX records to optimize delivery speed.
What is an Internal Relay domain in Exchange Online?
An Internal Relay configuration tells Exchange Online that it does not host all mailboxes for a specific domain. If a mailbox is not found within the Microsoft 365 tenant, Exchange will forward the message to an external server using your configured mail flow connector.
Can I migrate mailboxes gradually using this method?
Yes. By setting the domain to Internal Relay and configuring an outbound connector, you can migrate mailboxes in batches. Users already migrated to Microsoft 365 receive mail internally, while remaining users have their mail routed to the old server.
