Email marketing is an effective way to keep up with potential and past clients and make sure they know what you are doing now in your business. It's also used by many large companies to promote coupons/weekly specials.

As php newsletter script in the email marketing ,we usually inserted a small picture or sentences in the email to make which invisible. The PHP function will give back the echo from the small pictures to return the tracking status to the your mail server. When you open the newsletter, and allow to appear all pictures in the email, this tracking function will be activated.