Hey Jay-
You can go into a lead's activity log & type the word delivered, and manually count the number of emails delivered in a week.
If you previously had thought of this potential, you could have created a smart campaign, everytime an email is delivered, increase the "Emails Delivered" Custom Score by +1. Then you'd have a record of how many emails each person had had delivered. but if you haven't already set that up, you can't do it retroactively.