An email marketer friend of mine received a note from a customer the other day. It turns out that one of her recent data-appending projects had matched the wrong person. Paraphrasing, the customer's note said, "Please remove me from your list. My name is Mary, not Angela. In case you want to know, Angela is the woman who had a two-year affair with my husband, convinced him to divorce his wife of 12 years (me), and turn her children against their mother. Angela now lives in my house with my children and my husband. Perhaps you should try her email address instead of mine."

Data appends are a critical weapon in the marketer's arsenal, but they do not always hit their mark. Append services vary in the degree of effort they undertake to match an email address to offline data (e.g., name, address, etc.). Some services match on name, while others go further, matching address and household. A few even offer matches at an individual level. While individual-level match rates often are lower, individual-level matching may prevent Mary from receiving any more of Angela's email.