Retire last Exchange Server but keep directory sync

Honestly, you might want to look for a partner to decommission your last Exchange server. You can unintentionally wipe the Exchange attributes on your AD User accounts that you'll still need. Also, it can be a bit tricky to manage things like proxyAddresses without an Exchange server.

If you are going to move forward yourself, here are some docs:

• https://learn.microsoft.com/en-us/exchange/manage-hybrid-exchange-recipients-with-management-tools

• https://www.alitajran.com/remove-last-exchange-hybrid-server/