You need to disable the account in AD. Then in the exchange admin center, find the users mailbox and enable forwarding. There should be an option to forward which does not keep a copy in the mailbox, and then forward and keep copy in the mailbox.
If the mailbox gets tons of email, or you want to see emails currently in the mailbox, you can delegate read access to another user. That user can then add that former employees mailbox to their Outlook as a separate mailbox.
You have to be an exchange administrator to do this.
Learn about exchange admin center.
Https://learn.microsoft.com/en-us/exchange/architecture/client-access/exchange-admin-center?view=exchserver-2019
Learn about managing mailbox including forwarding.
https://learn.microsoft.com/en-us/exchange/recipients/user-mailboxes/user-mailboxes?view=exchserver-2019#mailbox-features