Data append is a reverse-email append. So, in order to understand what it is, first you need to be familiar with the term email append! Basically, email append is a common marketing technique, which involves using customer info (full name, address, phone number) and comparing it against vendors database in order to obtain the email address of the customer. Data append is a totally opposite process it involves taking the customers email address and using it to learn more information about them. For example, this way, you can find out when their birthday is, so that you can send a greeting email.