Going forward you can also create smart triggered campaigns to capture people who have opened/clicked email,. You can create three score fields ( Email opened/ Email Delivered/ Email Clicked). Each time you send an email to any of the record and any activity is performed it will keep adding a score of +1 to the field.
As an example:
Lead A is sent 10 emails and Lead B is sent 12 emails,
Open for Lead A is 8 and B is 6.
This way you will keep on tracking the details and you can run a smart list to see the details at once.
This is not a good idea. If your email volume is high, this smart campaign will be ruining almost always. This will eat up the marketo server resources and might block/delay other smart campaigns execution. Remember, only 3 smart campaigns can be in execution at the same time.
How will you find out which email has been clicked or opened? lets say you do this activity for 2 quarters, post that how will you identify who does what activity on which email?