Read what a free tool claims to check before judging its output. A tool reporting that a domain has valid MX records has told you something true and narrow: mail could be delivered to that domain. Whether the mailbox exists is a separate question it did not ask.
Where free tools mislead is in presenting a DNS result as a verdict. A green tick meaning the domain resolves looks identical to a green tick meaning the mailbox was confirmed, and only one of those was checked.
Free tiers from paid vendors are a different thing entirely. A monthly allowance of real verifications runs the complete pipeline on a small volume, which is a genuine sample rather than a reduced check. Ours is 100 credits a month with no card.
A free service offering unlimited full-pipeline verification raises a question about revenue. The SMTP stage costs real money to run, and where the service is free at scale the uploaded list is frequently the product.
For a handful of addresses, a free tool with a genuine SMTP check is perfectly adequate. For a list you are about to mail, the thing to compare is not price but what share of the addresses came back unresolved.
Five test addresses that expose a domain-only tool
You can find out what a free tool really checks with five addresses. Use your own working address. Add a mailbox at your company that was closed last year. Make up a long random name at gmail.com. Then add an address at a domain with no mail servers, and finish with one that has a typo in the domain.
A domain-only tool gets the last two right and fails the ones that matter. It approves the closed mailbox, because your company's domain is alive. The random Gmail name gets approved for the same reason.
A tool that runs the mailbox check rejects the closed mailbox, since your server says no such user. On the random Gmail name, the honest result may well be unknown, because Gmail often blocks these probes. If a free tool returns a confident valid for a name you invented ten seconds ago, you've learned all you need to know about it.
A green tick has four possible meanings
The same green tick can mean four different things. It might mean the address is shaped correctly. Perhaps the domain exists, or the domain has mail servers. Or it might mean a mail server accepted that exact recipient. Only the last one says anything about the mailbox.
You can tell which you're getting from the output. Look for a server reply code such as 250 or 550 next to the result. A tool that talked to a mail server has one to show. Catch-all and unknown verdicts are worth looking for as well.
That second clue is the strongest. Run twenty business addresses through the tool. On a typical business list, around a quarter of addresses sit on domains that accept everything, and some servers won't answer at all. If a tool never says catch-all and never says unknown, either it isn't holding the conversation or it's guessing at the end of it.
Related questions
Are free verifiers accurate?
Exactly accurate on syntax and DNS. Most skip the SMTP stage, which is the part that confirms a mailbox.
Can a free tool confirm a mailbox exists?
Only if it runs a real SMTP check. Many report DNS results in a way that looks like confirmation.
Is a paid vendor's free tier useful?
Yes. It runs the full pipeline on a small allowance, so the results are directly comparable to the paid product.
Why be cautious about unlimited free verification?
The SMTP stage costs money to operate. Where a service is free at scale, check what its terms say about your data.