How to Clean Up Old Exchange Attributes in Active Directory?

You shouldn't try to clean up all those old attributes. It’s safer to keep them all intact. You can focus on cleaning up actual Exchange accounts instead. I found a helpful guide on the official documentation about managing hybrid Exchange recipients when shutting down your last on-prem server.

If you're looking for a way to start managing attributes, you might want to check out ManageEngine AD Manager Plus. It lets you rename and modify attributes, which could be helpful for your cleanup efforts. Just keep in mind that it's an on-prem solution and you’ll need a Windows server for it.

In a hybrid setup, you should avoid cleaning up the msExch attributes because they are crucial for Entra Connect Sync. They may look unnecessary, but removing them can disrupt recipient lookups and mailbox provisioning. If your aim is to fully transition to the cloud, consider using Microsoft's object-level Source of Authority feature instead of doing a manual cleanup. You’ll need to decommission your last Exchange Server properly instead of just deleting attributes. As for contacts and public folders, leave them alone unless remove them as part of the Exchange uninstall process if you are decommissioning.

My advice is to avoid deleting anything unless absolutely necessary. Altering attributes without a proven reason can lead to bigger issues down the line, so it’s better to leave things as they are for now.