Hey DJ,
There isn't a perfect way to do this, but you can at least set a "floor" for emails received by leads in your smart list.
You won't be able to see exactly how many emails each person got, but you can add the filter+constraints in the screen shot below to your existing smart list to know that, for example, everyone qualifying in your list was sent at least 5 emails in the past 180 days, or whatever the range is that you want.