A second mailing (both in direct mail and email promotions) to a specific audience within a reasonable time after a first mailing. Usually increases total number of orders.
Also, a sales letter sent to someone who has made an inquiry inviting the inquirer to make a purchase; part of the inquiry / lead conversion process. This process is generally used for expensive items requiring a lot of information and thought before a purchase decision is made. Follow-up letters differ from other promotional mailings in that they are mailed in response to an inquiry on an individual basis in contrast to being sent in bulk on a date determined by the advertiser. The first follow-up letter in a series is usually the most detailed, while subsequent efforts highlight individual benefits.