After spinning off a domain from Office 365, emails still routed back before sent out

We have a hybrid Office 365 setup with multiple domains and recently we spun off one domain to a standalone Office 365 tenant (which is not hybrid).

Let me call the domain remaining in hybrid setup as @remain.com and the spun off domain as @spunoff.com

We changed the MX record, remove domain from original Office 365 tenant, add the domain to new Office 365 tenant, migrated all mailboxes accordingly and updated the Address Book. Everything work fine and running smoothly. I thought it was a complete success.

I have also removed @spunoff.com as UPN Suffix from our Active Directory which is not really using remain.com but just use local domain like remain.local

But recently when I reviewed the send and receive logs of our on-prem Exchange 2013, I was surprised to see some strange things.

What I found is that emails sent from Cloud mailbox of @remain.com to email address of @spunoff.com are all delivered from Office 365 to our on-prem Exchange 2013 before sending back out to Office 365 again.

This situation does not happen to other domains sending to @spunoff.com. For example, I sent a testing email from Gmail to @spunoff.com , there is no sign of having the email going through our on-prem Exchange 2013 before sending out . This can also be confirmed by inspecting the delivery transcripts of the delivered email.

And this also does not happen when sending a testing email from the @remain.com Cloud mailbox to Gmail. That email did not go through our on-prem Exchange 2013.

The reverse path is also OK - emails from @spunoff.com to @remain.com Cloud mailbox do not go through our on-prem Exchange 2013.

But then why emails sent from @remain.com Cloud mailbox to @spunoff.com being directed through our on-prem Exchange 2013!?

Is there any reconfiguration still missing? Where should I double check?


Regards

Andrew
Andrew ChingAsked:
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Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
"But then why emails sent from @remain.com Cloud mailbox to @spunoff.com being directed through our on-prem Exchange 2013!?"

Where is the send connector pointing? Are you familiar with this type of configuration ie. send connectors receive connectors etc?

When you send a mail from@remain.com cloud mailbox are you certain its not connected to the on prem exchange server?

When you send a mail from@remain.com cloud mailbox to randomdomain.comlikegmail does the email go through the on prem exchange server?
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Andrew ChingAuthor Commented:
Hi Mark,

Good Advice!

I took a look at the Office 365 Outbound Connectors and indeed it still specified that emails for spunoff.com plus other remaining domains should be routed via our organization's email server.

The initial hybrid setup wizard did the job so well that I overlooked I need to manually work on Connectors now when spinning off domain.

I've removed spunoff.com from the Outbound Connector. My first testing indicated spunoff.com emails still routed via our server BUT I believe I just need to wait longer for the change to be effected in Office 365 environment.

I'll check the routing again after a day before confirming this.

Crossing fingers...

Andrew
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Andrew ChingAuthor Commented:
Hi Mark,

You are exactly right. By removing the spunoff.com from Office 365 Outbound Connector, those emails no longer go through our on-prem server.

Best Regards

Andrew
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Andrew ChingAuthor Commented:
Thanks for the guidance!
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Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
np happy to help #1
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Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
is it fully resolved now?
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Andrew ChingAuthor Commented:
Hi Mark,

It is fully resolved. No more spunoff.com emails routed via our on-prem server.

Reconfiguration Office 365 Outbound Connector is really what I missed.

Regards

Andrew
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Mark BillExchange, AD, SQL, VMware, HPE, 3PAR, FUD, Anti MS Tekhnet, Pro EE, #1Commented:
Nice job! GL
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