Reserve Email Addresses for old users within Exchange organization

Due to a software dependency, I have an odd requirement in my environment, while trying to clean up stale accounts. I want to remove all the stale AD Users and Exchange mailboxes from the environment, however, I need to somehow reserve the SMTP Address for the stale account, so that the address cannot be assigned to another mailbox later (maybe a new user with the same name). I'm looking for the best strategy. technical details are not my concern.

I've ended up with the below two strategies (I think still there must be a better way, please advise):

1. convert each mailbox to a contact. Steps:

   • remove mailbox (this will also delete the AD user)
   • create a contact with the email address of the deleted mailbox Cons: not much cleanup! because each object is actually replaced by another object.

2. preserve all email addresses by adding each as a proxy address for a dummy user. Steps:

   • remove mailbox (this will also delete the AD user)
   • add the email address of the deleted mailbox as a proxy (additional) address into a dummy mailbox Cons: I have more than 6000 stale mailboxes. in exchange 2019 you can have ~1000 addresses per object.

another approach might be a "Disable-and-then-Enable-mailbox" strategy, which will remove all the old mailbox contents, preserves the address, and also preserve the user account. like the <1> method mentioned earlier, it actually does not clean up much.

What do you think is the best way to preserve a lot of email addresses in the Exchange Server (or Active Directory), and avoid those addresses from being assigned to new users after the original mailbox is deleted?