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 an Opt-In List, Building Relationships
If you aim for success, you must do everything within your means to achieve that. You just donÃt go on sitting there in your house complaining about not getting as much profit as you expect.