Create an inactive mailbox
*Mailbox must have a license and be active before you can make it an Inactive mailbox.
1. Log in to the O365 Admin portal.
2. Under Admin Centers select Security & Compliance.
3. Click on Data governance and select Retention.
4. Select the policy you want to add a user to.
5. Click on Edit policy and select Locations applied.
6. Select Choose recipients from the Exchange email option.
7. Select Choose recipients and search for the user you would like to add. Once found click the check box and click on Choose.
8. Click on Save.
9. Remove license from users mailbox and you can now disable the account in AD.