Infrastructure explains more disagreement than anything else. A receiving server that answers one connecting address honestly may throttle or refuse another, so two services running identical logic can get different answers from the same host.
Retry behavior explains most of the rest. Greylisting returns a temporary rejection on first contact by design. A verifier that retries later gets the real answer, while one that scores the first reply may mark a good address as bad.
The catch-all probe is the disagreement that matters most. If one service tests whether the domain accepts an invented address and the other doesn't, the first reports catch-all and the second reports valid, and the second learned nothing. Can catch-all emails be verified shows that probe step by step.
Labels cause conflicts that aren't real. NeverBounce's developer docs, read on 24 September 2026, list five codes: valid, invalid, disposable, catchall and unknown, and describe catchall as "a domain wide-setting, also known as Accept-all (Unverifiable)". ZeroBounce scores catch-all addresses on a 1 to 10 scale, read on its own site the same day. So when ZeroBounce's report says valid and NeverBounce says catch-all, check whether the first tool's valid came from its catch-all score. If it did, both tools found the same domain setting and named it differently.
Timing is the dullest cause. Two checks a week apart can differ because a mailbox closed in between, and neither service got it wrong.
When two services disagree, read the raw response instead of counting votes. A verifier that shows the server's own reply lets you judge. One that shows a bare verdict asks for trust. ZapBounce puts the literal reply code on every row of your results, so you can settle a conflict from the evidence.
A 1,000-address disagreement, sorted into four piles
Say you run the same 1,000 addresses through two services. They agree on 860 and differ on 140. Don't count votes. Sort the 140 by what kind of disagreement each one is.
Suppose 70 are valid at service A and catch-all at service B. That's the probe difference. B tested the domain with a made-up address and A didn't, so believe B. Another 40 are unknown at A and invalid at B. Look at B's raw replies here. If they're temporary 4xx codes scored as dead, B is wrong and those 40 people may be real.
A further 20 differ in label only, risky at one and catch-all at the other. Those aren't real disagreements. The last 10 changed between runs for the dullest reason there is: the two checks happened a week apart and a few mailboxes closed. Out of 140 conflicts, only the 40 needed a judgment call.
When the disagreement matters, and a rule for ties
For a single campaign, a small disputed pile isn't worth an afternoon. If the two services differ on 2% of your list, put those addresses in a separate small send and let the bounces settle it. You'll have the answer by tomorrow.
It matters a great deal when you're picking a vendor, because the disputed pile shows you each one's habits. A service that turns temporary failures into invalid will keep deleting real people from every list you give it. One that skips the random-address test will keep passing unproven addresses as valid.
When you have to break a tie with no raw reply to read, don't take the sunnier verdict by default. Between valid and catch-all, treat the address as catch-all. If it's invalid against unknown, go with unknown. Choosing the cautious label costs you a separate send. The hopeful one costs you a bounce or a lost customer, and you won't know which until later.
Related questions
Why does ZeroBounce say valid but NeverBounce says catch-all?
Usually because the domain accepts every address, and the two tools label that finding differently. NeverBounce reports it as catchall. ZeroBounce scores catch-all addresses from 1 to 10, so check the score behind its valid before you trust it.
Which email verifier should I believe?
Believe the one that shows the server's actual reply. A raw 250, 550 or 4xx code lets you judge the verdict yourself, while a bare verdict asks you to take it on trust.
Does disagreement mean one verifier is broken?
Usually not. Different IP standing and different retry rules produce different answers from the same server, and label differences account for much of the rest.
Should I run a list through two services?
Once, as an evaluation, it's useful, especially to compare the share each one declined to judge. As routine practice it doubles the cost for little gain.
Why do labels differ so much between vendors?
There is no standard vocabulary. Risky, accept-all, unknown and catch-all overlap differently at every vendor, so compare what each label means before comparing counts.