It is considered a best practice to establish a retention policy and configure each message item type's Maintenance options accordingly. In addition, you must also create Unity Scheduler tasks to perform the actions of archiving, purging, or deleting message items.
Message items should be archived or purged when possible to maintain system performance. Non-archived message items are only intended to be stored within the database for a short period of time for diagnostics purposes; they are not intended to provide a permanent record of the transaction.