What is email list hygiene?

B2B contact data decays continuously, and not because anyone did anything wrong. People change jobs and their mailbox is closed. Companies rename or consolidate domains. Individual mailboxes are retired while the domain carries on. None of that produces a notification; the first you hear of it is a bounce.

We do not publish a decay rate, and you should be suspicious of anyone who does without saying how they measured it. What is checkable is the direction: a list verified six months ago is meaningfully worse than one verified this week, and the cost of finding out by sending is paid in sender reputation rather than in credits.

Remove hard bounces immediately and permanently. A 5xx rejection for a missing mailbox is the receiving server telling you the address is gone. Retrying it is the single clearest signal that a sender is not reading its own bounces.

Suppress, do not delete, unsubscribes and complaints. A deleted address can be re-imported from another source next quarter; a suppressed one cannot. This is the part list hygiene shares with compliance, and it is the part that is hardest to undo when it goes wrong.

Re-verify before a send, not on a calendar. Verification is a statement about the moment it ran. The useful question is not "how old is this list" but "how long ago was this address confirmed, relative to when I am about to mail it".

Segment the addresses that were never confirmed. Catch-all domains and unknown results are not bad addresses; they are unmeasured ones. Mixing them into a bulk send makes the whole send unmeasured.

The honest answer is that it depends on the source and the gap since the last send, not on a fixed interval. A cold or purchased list should be verified immediately before every send. A warm list that you mail weekly and whose bounces you process properly needs far less, because the sending itself is doing the measuring.

The practical rule: verify before any send to a segment you have not mailed recently, and before any send large enough that a bad batch would matter. If you are about to mail an address you last confirmed a year ago, you are guessing.

Not a bounce rate. Google’s sender guidelines ask senders to "keep spam rates reported in Postmaster Tools below 0.10% and avoid ever reaching a spam rate of 0.30% or higher", and to follow the bulk-sender requirements if you send more than 5,000 messages a day to Gmail accounts. Those are complaint rates, not bounce rates — the distinction is covered on bounce rate.

List hygiene moves both numbers anyway, for the same underlying reason: mail that reaches people who expect it generates neither bounces nor complaints.