Which of the following actions signifies that an email mailbox is permanently removed?

The action that signifies an email mailbox is permanently removed is deleting the mailbox in a tactical environment. This process typically involves not just deactivating the mailbox temporarily but rather initiating a complete and irreversible deletion of the data associated with that mailbox. In a tactical environment, such deletion means that the mailbox is no longer recoverable, and all associated emails and settings are lost permanently.

Reassigning the email to a new user keeps the mailbox intact; it simply transfers the ownership or access rights. Flagging the account for review indicates that it is under scrutiny, not that it is being removed. Temporarily disabling the account implies that it can be reactivated later, which does not result in permanent loss of the mailbox or its contents. Thus, deleting the mailbox is the only action that definitively concludes the existence of that mailbox within the system.