Consider what each stage costs to run. A syntax check is a regex. A DNS lookup is a cached query costing fractions of a cent. An SMTP conversation needs a connection from an address that major providers will accept, and that address is a recurring expense that has to be maintained.
Port 25 is the constraint nobody advertises. Residential ISPs block it, Google Cloud blocks it outright with no exception process, AWS requires a justified request, and Azure blocks it for most subscriptions. Running the decisive check means having infrastructure where the port is open.
Reputation on that infrastructure decides how many servers answer. A connecting address with poor standing gets throttled or refused, which converts answerable addresses into unresolved ones. Two services running identical code produce different coverage for this reason alone.
Free tiers from paid vendors are a different proposition from free tools. A monthly allowance of real verifications is a sample of the full product, and ours is 100 credits a month with no card.
A genuinely free full-pipeline verifier for large volumes should prompt a question about the business model. Where the service is free at scale, the list is frequently the product.
Try opening port 25 from your own laptop
You can feel the constraint in about ten seconds. Open a terminal at home and run nc -vz gmail-smtp-in.l.google.com 25. On most home broadband, the command hangs and then times out. Your provider blocks outbound traffic on that port to stop infected home computers from sending spam.
Try it again from a cloud server on one of the big platforms, and you'll often get the same result. Those platforms block or restrict the port for the same reason.
That locked door is what a free browser tool runs into. The page can check the shape of an address, and it can ask a lookup service whether the domain has mail servers. It can't hold the conversation that asks about the mailbox. That needs a machine where port 25 is open, and whose address mail servers are willing to answer.
What a domain-only check lets through
Think about what a domain check says for the biggest providers. The domain gmail.com has mail servers and always will. So every Gmail address that was ever closed, mistyped or invented passes a domain check. The same holds for outlook.com and for every company that's still in business.
Say you mail 20,000 addresses after a free domain-only pass. The check removed the dead domains, which might be 2% of the list. It left every dead mailbox at a live domain, and those make up most of the decay. You'll find them in your bounce report.
Free is plenty in some cases. Checking three new client contacts before an invoice run is one, and a shape-and-domain check on a form field is another. It stops being enough at the moment you're about to send a campaign, because that's when a wrong answer costs you reputation.
Related questions
Are free email verifiers accurate?
For syntax and DNS, yes, because those checks are deterministic. Few run the SMTP stage at volume, which is where the real answer lives.
What does the paid tier actually pay for?
SMTP infrastructure with port 25 access and maintained IP reputation, plus the engineering for retries and catch-all probes.
Is a free tier from a paid vendor useful?
Yes. It runs the full pipeline on a small allowance, which is a real sample rather than a reduced check.
Why should I be cautious with free bulk verifiers?
The economics do not work without another revenue source, and the list is often it. Read what they say about data handling.