The act of maintaining a list so that hard bounces and unsubscribed names are removed from mailings. Some list owners also use an email change-of-address service to update old or abandoned email addresses (hopefully with a permission step baked in) as part of this process.
Building Your Business with an Opt-in List
Before revealing the secrets of the trades, here are myths and fallacies that need to be cleared before one indulges into building an opt-in list.