Having an issue with the receive connector in our hybrid Exchange 2016 environment. Successfully ran through the HCW and have migrated a test mailbox. I can send/receive internally to that mailbox and email from an external address to it also. What I can't do is email from the test mailbox to external addresses. That results in the Remote Server returned '550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain' message.
I have read various articles and it looks like the Default Frontend receive connector is set correctly but my on-prem environment isn't using it. There is already a custom receive connector listening on port 25 for SMTP relaying for network devices. I'm sure this is what the issue is but not sure how to move forward as they requirement for that connector is there.