When a Quickmail block is removed from a course page, all of the emails sent from within that course are deleted from the database. When Quickmail is a sticky block, removing it from the sticky blocks page deletes all (or at least many) of the emails from the database.

Because blocks get accidentally deleted, or need to be removed and re-added for various reasons, it would be more convenient to keep old emails in the db table. Even if the table grows fairly large this shouldn't significantly impact Moodle's performance.