This value is tied to the 'Sent' status reported by Blackbaud's email servers.  Once all messages have been successfully sent (or not sent) by Blackbaud's email servers, this number will no longer update.  It should be noted that opened/unopend statistics on the 'Messages Sent' tab will continue to update when sent messages are opened, and that the 'last updated on' text in the 'Breakdown of Messages Sent' is specific only to the email being sent from our servers.  (Not of the last update from an opened message)