HOW DOES IT WORK?
* Open rate: who opens the email.
* What customers do with the email: delete, spam, or click a link (aka click through rate).
* Results: depending on who opened the email and what they did with it, we can use that info to resend similar info to the people who opened it and clicked on an ad, change the content of our email if someone didn't open it & delete email addresses of people who marked our email as spam (so we don't email them again)