When you have moved all your mailboxes to the cloud, it's finally time to remove your on-prem Exchange servers and your Hybrid configuration. But before this happens, you'll need to make sure no applications relay mail via Exchange.

In my case, there were about 40 apps still relaying mails, so these had to be moved to High Volume Email (HVE) or where the application didn't support this we used an on-prem Postfix server.

With that out of the way we are left with the following tasks:
- Update the AutoDiscover record to point at O365.
- Remove oAuth both on-prem and in EXO.
- Remove intra-org connectors in Exchange.
- Move smtp-relay DNS record to point to Postfix server (for any apps that weren't already moved).
- Remove SCP points.
- Update or remove internal DNS records used for Exchange.
- Remove Kerberos ASA account from AD if it exists.
- Remove DAG and cluster objects if this is configured.
- Move any system mailboxes to a remaining Exchange server.
- Uninstall Exchange 2016 and reboot.
- Update scripts so that they no longer use the decommed servers.
- Remove federation cert - ADSI Edit may be required