Mail flow after removing hybrid configuration

Question

I have some questions about what happens with mail flow after removing hybrid configuration...

We are currently in hybrid mode. Exchange version is 2016. Mailboxes and public folders have been migrated but we are planning to use an Exchange on-prem server for SMTP relay for internal LOB applications and scanners. We have a send connector in Exchange for routing mail from Exchange to Exchange online. The address space for this connector is company.onmicrosoft.com and company.mail.onmicrosoft.com and everything works perfectly. Mail addressed to external recipients go out from Exchange and mail to internal recipients gets routed to Exchange online using the remote routing addresses.

If we were to remove the hybrid configuration, would this continue to work as it currently does? Would emails to internal people still get routed correctly or does removing hybrid configuration also remove the remote routing addresses?
Would the recommendation in this scenario be to leave hybrid in place? What are the pros and cons of leaving hybrid mode versus removing it? If we were to remove it what, if any, additional configuration would be required to ensure that email continues to get routed properly from Exchange to Exchange online?

Thanks a lot,
Kenny

Answer 1

Removing the hybrid configuration wont remove the remote routing addresses. Those are tagged on the remote mailboxes on-prem and as long as those and the send connectors remains mail flow will continue to work as it does now.

However, there is no real cost to just leaving the hybrid connection in place. In fact, Microsoft recommends you do not remove it all in your scenario:
https://learn.microsoft.com/en-us/exchange/decommission-on-premises-exchange#scenario-three

If you are using AADConnect, you dont want to remove it neither.