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?