How can i route specific domains to specific mail servers?

calgia 21 Reputation points


I'm starting to integrate a existing Domain to our Microsoft 365 tenant.

The Domain as example "", from a brand, is currently registered at our Webhoster as example "Hostprovider". The Mailserver is currently at "Hostprovider" and comming witht the server. The Current MX Record is as example "".

Currently a other Domain as Example "", from the main company and brand, is registered to the existing tenant with url as example "".

If i integrate the Domain "" to the existing tenant "" without the mail exchanger, to setup everything before migrating, then if someone with a "" sends some mail, Microsoft 365 see that the domain is on the Microsoft 365 tenant and wanna try to send it to the (eventually) existing postmailbox "", it answers with some not found message.

It's possible to add some connectors.

I tried it but i got a "error" message at step 3.

How can i successfully integrate, also without a postbox at the 365 for "" that a mail to "" is send to as long the integration is not fully completed?



Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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  1. Mike Hu-MSFT 2,180 Reputation points Microsoft Vendor


    Welcome to the Microsoft forum.

    Follow the steps:

    1. Add domain “” to Microsoft 365 in EAC: Add a domain to Microsoft 365 - Microsoft 365 admin | Microsoft Learn
    2. Create a new connector in EAC:
    • EAC->Mail flow-> Connectors->Add a Connector
    • Connection from “office 365” , Connection to “Parter organization”
    • Use of Connector choose “Only when I have a transport rule set up that redirect to this connector”
    • Specify the domain ( and the mail server (
    1. Create a Mail Flow Rule (Transport Rule)
    • EAC->Mail flow ->rules->Add a rule
    • Apply this rule if the recipient’s domain is “
    • Do the following: Redirect the message to the following connector “The connector you created”

    Hope this helps. Please feel free to contact me for any updates.

    feel free to contact me for any updates.