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Office 365 - Exchange 2010 Hybrid configuration mail flow issues

We have configured the Hybrid connector between our Office 365 location and Exchange 2010 server and this has completed without error.

However if we e-mail an e-mail address that only exists in one location then we get an NDR from the other (e.g. e-mail address exists on a mailbox in Exchange 2010, we try e-mailing this from Office 365 mailbox and get an NDR from Office 365 that the address does not exist).

If we set a domain to only exist in one location (e.g. the domain only exists in Exchange 2010) and configure this domain if Office 365 to be the 'Internal Relay' type then mail flow works as expected and this can be delivered.

I believe that we are missing something with the Office 365 portal configuration to allow domains to be configured as the 'shared' type (there is no option for this on the 'accepted domains' tab).

Any suggestions on what we might be missing would be greatly appreciated.
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8/22/2022 - Mon
Vasil Michev (MVP)

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Thanks for the help Vasil. Your solution mixed with changing the DNS server fixed the issue
I started with Experts Exchange in 2004 and it's been a mainstay of my professional computing life since. It helped me launch a career as a programmer / Oracle data analyst
William Peck