To be clear, this information is not available in SQL - such information would have to come from the message store itself (Domino or Exchange in the case of Unity - for Connection it's a different story).
The limitations of the Exchange notification triggers (i.e. often we get only "hey, something changed in this inbox") and nothing more leaves us with limited options for keeping track of exactly what messages arrived for who/when. The message activity report for subscribers gives a limited view on the number of messages that we find when we get notification events and when users call in, but it's not broken down and we don't get a complete laundry list.
this gets especially tricky when Unity conversation is not directly in the line of fire when a message comes through (i.e. remote networking, desktop message activity for sending/reviewing/deleting etc...).
You can, of course, break down current message space utilization using the Message STore Manager, but for reconstructing a detailed history of how many messages of what type and from whom were left for individual users over a period of time, that's more difficult. There are likely still 3rd party tools that run against Exchange diag logs (there were a few years ago when I went out to research this - not sure now, I'd have to dig into it again) for message history type information. Something like this would be necessary for anything close to an accurate/detailed view of actual message traffic.
