There arent any recommended practices really. But if you remove the Exchange Online license, the mailbox will be removed and senders will get a notification that the email address is not valid.
How to disable a user's mailbox in Microsoft Online Exchange to reject emails
Rita Bautista
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What are the steps to disable a user's mailbox in Microsoft Online Exchange to make sure that emails sent to this mailbox are rejected with a notification that the mailbox no longer exists? Please provide details and recommended practices.