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I currently have a domain on Office 365 and I am adding a second one. However, the moment I added the new domain and verified ownership by adding the TXT record, my users on the first domain can no longer send e-mails to the second domain. They simply bounce, though I have not updated the MX record.

This is not the behavior I want. I want to create the user accounts first, get them set up for my users and then flip the MX switch. Until I do, new mails from the first domain should go to the current e-mail server and NOT Office 365.

I have found various suggestions for solutions that involve setting the secondary domain up as an "internal relay domain", but I am always met with a warning in the Office 365 ECP, that goes:

An outbound connector to route this domain can't be found. Make sure that there's an Outbound Connector of the type OnPremises that matches every non-authoritative accepted domain. The connector can either have the AllAcceptedDomains flag enabled or have a recipient domain that matches the accepted domain.

This error is cryptic to me. I have tried setting up an outgoing "partner" connection that should pick up the MX records, which also triggers an error, but it doesn't work.

Any mail sent from my first domain will be rejected by Microsoft's mail servers and never make it to the current mail host for the domain.

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Edit:

One thing I do know to check for, is that your 2nd domain is an actual domain with mailboxes rather than just Office 365 user IDs with email contacts; meaning, you have to have actual Exchange subscription plans and mailboxes set up for this 2nd domain and not just an Office 365 user ID. That is probably why you can't find an outbound connector.

If that doesn't work, I hate to be vague in my answers, but I will tell you this: Call Microsoft and have them help you. You get support for having your Office 365 subscription and they are very prompt about getting an engineer to you.

Also, the reason I can't tell you what to do specifically here, is that when working with MS support on this very issue for a client of mine, we had to have Microsoft flip "magic switches that only they can flip" to get things to work right. It definitely has to do with them both being on the same O365 account, but you will need to work with them to get it set up properly until you edit your MX records.

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