List hygiene

What it covers

List hygiene is the routine upkeep that keeps an outbound list from degrading into a liability. It has four parts:

1. Verify before sending. Confirm each mailbox exists. See email verification.
2. Suppress permanently. Every hard bounce, unsubscribe, and complaint goes on a suppression list that every future import is checked against.
3. Re-verify on a schedule. Lists age on their own; see data decay.
4. Prune the silent. Stop mailing addresses that have never engaged after a reasonable number of attempts.

The part people skip

Suppression only works if it spans everything. A person who unsubscribed from one campaign and is re-imported in another from a fresh CSV gets mailed again, which is both a complaint risk and, depending on where they are, a compliance one. Keep a single suppression list across tools and import paths.

How often to re-verify

There is no universal interval, since decay depends on the audience. People change jobs and companies restructure addresses constantly. As a rule: re-verify before any send to a list that has been sitting for a few months, and sooner in sectors with high turnover.

What it is worth

Hygiene is the cheapest lever on bounce rate and the main defence against spam traps. Neglecting it is how a domain with good content ends up in the spam folder. See what a high bounce rate costs.