Contact list hygiene is the regular cleaning of a database to remove addresses that harm deliverability. Companies tend to collect contacts and never remove them, which is a mistake: invalid addresses cause hard bounces and signal that the list is not maintained, and long-term inactive subscribers lower the open rate, by which the sender's reputation is judged. Cleaning includes immediately removing non-existent addresses, dropping contacts who have not opened any message in a year, and fixing typos in domains. Before dropping inactive contacts, send them one re-engagement message with a clear question about whether they want to continue. After cleaning, the list tends to be smaller and the results better, because the messages reach where they are meant to. Never buy address databases.
See also: Email Deliverability, Double Opt-in, Win-back campaign.
