What can and cannot be verified
Three things are checkable without sending anything. Syntax: whether the address is validly formed. Domain: whether it resolves. Mail servers: whether it publishes MX records, meaning something is prepared to accept mail for it.
Whether a specific mailbox exists is not reliably checkable, and this tool does not claim to. That requires an SMTP conversation on port 25, which cloud hosts block, and even where it is possible the answer is often wrong -- see below.
Why mailbox verification is unreliable
The traditional method opens an SMTP session and issues RCPT TO for the address, reading whether the server accepts it. Large providers have made this useless on purpose.
Gmail and Outlook accept RCPT TO for addresses that do not exist and discard the message afterwards, specifically so spammers cannot enumerate valid addresses. Many servers also employ greylisting or rate-limiting that produces temporary failures indistinguishable from a rejection. A service claiming high-confidence mailbox verification against major providers is overstating what the protocol can tell it.
Role accounts and why they matter
Addresses like info@, support@, sales@ and admin@ are role accounts: shared mailboxes tied to a function rather than a person. The tool flags them because they behave differently.
Many email marketing platforms refuse to send to role accounts, since they attract complaints and often reach several people who never individually subscribed. For transactional mail they are perfectly fine. For a mailing list signup, a role address is worth a second look.
A domain with no MX records
If a domain publishes no MX records, most senders will not attempt delivery. RFC 5321 permits falling back to the A record, and a few servers still do, but you should not rely on it.
A domain that does not resolve at all cannot receive mail under any circumstances. That is the one unambiguous negative this tool can give you, and it usually means a typo in the domain part.
Frequently asked questions
Can you tell me if this address really exists?
No, and neither can anyone else with certainty against the major providers. This tool verifies syntax, domain and mail servers, and says plainly that mailbox existence was not checked.
Is it safe to send to an address that passes these checks?
It means the domain can receive mail. Whether that particular mailbox exists is unproven, so expect some bounces when validating a list this way.
How should I clean a mailing list then?
Send to it and process the bounces, which is the only definitive test. Remove hard bounces immediately; repeatedly mailing dead addresses damages your sending reputation.
Does checking an address here notify its owner?
No. Only DNS lookups are performed. No mail is sent and no connection is made to the mail server.