Email lists change while you aren't looking. People leave jobs, broadband contracts end, and whole domains switch to accepting all mail. Hunter measured it in an eight-week experiment published on 4 November 2025: 5.5% of addresses that had passed verification degraded, 2.3% to invalid and 3.2% to accept-all.
That's why the question isn't whether to verify. It's when. Five moments cover almost every list, and each one is cheap next to a bounce spike on a campaign that matters.
Start with capture. A signup form that checks the address in real time stops typos and throwaway domains before they reach your database. ZapBounce's API answers one address at a time with POST /v1/verify, and its did_you_mean field catches a mistyped domain while the person is still on the page.
Next comes any large send to B2B data more than eight weeks old. Staff turnover drives business-list decay, and Hunter's figure covers exactly that window. A bulk check before the send removes the addresses that left with their owners.
Three more moments, and a cadence by list type
Before a migration. Moving to a new ESP or new sending domains means you start with no reputation, and your first sends set it. Clean the list before those first sends, not after the bounce report.
Before you mail a list that has sat idle. An annual appeal or a re-engagement campaign goes to data that has aged for months. Decay has built up in silence, and a check first keeps it off your new campaign.
After every import or purchase. A list from a data provider, an event or a partner arrives with no history in your system. Verification shows how much of it is reachable before you decide anything else. Your lawful basis to email those people stays your job, since a verification result never creates consent.
On a cadence. Re-check B2B data before any big send once it passes eight weeks. For consumer lists, once a year is enough, and ISP-heavy lists need it more often, because those mailboxes close when the broadband contract ends. On your next report, the catch_all share tells you how many domains switched to accepting everything since your last pass.
Pair each check with your engagement data
The short version
- Check addresses at signup with the real-time API so bad data never lands in your database.
- Re-verify B2B data before any large send once it is more than eight weeks old.
- Verify before a migration, before an idle-list send and after every import or purchase.
Questions people ask
How often should I verify my email list?
Verify B2B data before any large send once it is more than eight weeks old, and consumer lists about once a year. Add a check before a migration and after any import. In Hunter's November 2025 experiment, 5.5% of verified addresses degraded within eight weeks.
Should I verify a double opt-in list?
Yes, before a large send after a gap of a few months. Confirmation proves the address worked on the day it signed up. Job changes and closed accounts pile up after that, and a check before the send catches them.
Is it better to verify at signup or in bulk?
Both. Real-time checks at signup keep typos and throwaway domains out. Bulk checks before big sends catch the decay that happens after capture. ZapBounce runs both through the same API and the same billing rule.
What does it cost to re-verify a list?
ZapBounce costs $5 for 1,000 checks and $25 for 10,000, and credits never expire. Unknown results and duplicates are never billed. Every account gets 100 free checks a month with no card, enough to sample a list first.