EVERYONE wants to be able to do this. As I've said to these other posters with the same question as you (https://community.marketo.com/marketodiscussiondetail?id=90650000000PohbAAC, https://community.marketo.com/marketodiscussiondetail?id=90650000000PfioAAC):
Our workaround is to have a smart list that shows the email sent, by subject line--you get a count of how many were sent by each subject line. Then, pull into your smart list view the lead owner's last name so you can see who sent how many. The inherent problem is that if the rep changes the email's subject line, then you can't count that email!
I WISH MARKETO WOULD DELIVER ON THEIR PROMISE TO IMPROVE MSI REPORTING. They've been talking about it for years now...
A nice report, but what would make it better is to see the drill down to the contact level of those recipients/opens. Thx for the report doc link!