Short answer: every quarter, and immediately before anything important. Long answer: the right cadence follows from one number: email lists decay at roughly 2–3% per month. People change jobs (work addresses die with them), abandon old inboxes, and companies fold. Nothing you do causes it and nothing you do stops it; you only choose how much of it you're carrying at send time.
Why quarterly is the anchor
At ~2.5%/month decay, a quarterly clean means you're never carrying more than about 7–8% dead weight, which, caught before a send, costs you a small verification bill instead of a bounce event. Stretch to annually and you're sending into 25–30% rot: cliff territory. Clean weekly and you're mostly paying to re-verify addresses that were fine last week; that's what verdict caching is for, but it's still needless ceremony for most senders.
Quarterly also matches how reputation actually accrues: mailbox providers judge you on a rolling window of recent sends. A clean list every quarter keeps every window healthy.
The four exceptions: when "quarterly" becomes "today"
1 · Before the big send
Product launch, Black Friday, the annual everyone-gets-this campaign. Big sends concentrate risk: maximum volume, maximum visibility to mailbox providers, maximum cost if it goes wrong. Clean first, every time, even if the quarter isn't up. The send you can least afford to bounce is the one that most deserves a fresh verification pass.
2 · After any dormancy
If a list hasn't been mailed in 6+ months, treat it as unknown. Decay kept running while you weren't sending, and worse, some of those dead addresses may have been recycled into traps in the meantime. Reactivation campaigns onto uncleaned dormant lists are the single most common deliverability disaster we see.
3 · After the list grows sideways
Organic signups are one thing. But a CRM migration, an event-badge import, a merged brand's list, "the spreadsheet from sales": any bulk arrival of addresses you didn't collect yourself deserves verification before it touches your sender reputation. Every import inherits the hygiene of its worst source.
4 · At the first warning sign
A bounce rate that ticked up, open rates that sagged for no creative reason, an ESP warning email, a spam-folder report from a customer. These are symptoms of the same disease, and the diagnostic is cheap: run the list, read the verdicts. If it comes back healthy, you've ruled out the list in ten minutes and can hunt elsewhere.
The cost sanity-check: cleaning is priced per address in cents; a damaged sender reputation is priced in weeks of missed inboxes. On any list that makes money, the maths never gets close. And because verdicts are cached, re-cleans of a mostly-healthy list are the cheapest sends of all. You only really pay for what changed.
Make it automatic
The senders who never have bounce emergencies aren't more disciplined. They've just removed the decision. Put a recurring reminder on the first Monday of each quarter, or let the system do it: Scrubbed emails you a re-clean nudge when your last clean ages past 90 days, and connected ESP lists can be re-imported and cleaned in two clicks.
quarterly by default. today if it's big, old, borrowed, or acting weird ✦
The free evaluation checks a sample of your list end to end and tells you straight whether it needs cleaning. Verdicts in plain English, about ten minutes, no card.
Evaluate my list free →