Email marketing might be your number one source of traffic and income even above facebook ads. The data and marketing Association reported that for every dollar they spend on email they gained $38 back. There are only four metrics that matter though. They are deliverability, open rate, click through rate, and disengagement rate. Deliverability tells you what percent actually made it to the mailbox. Open rate is what percentage of emails were opened instead of just scanned by. Click Through rate is how often someone clicks a link in your email.