Syntax, then DNS
The local part must be a dot-atom of letters, digits and the common symbols RFC 5322 allows, at most 64 characters, and the whole address at most 254. Quoted local parts are not accepted. The domain must pass the same validation as every other DNSfinder lookup. A failure here ends the check with Invalid email address syntax and a score of 0.
Next come three DNS queries: MX, A and AAAA for the domain. If the MX query fails with SERVFAIL or a timeout, the tool cannot know whether mail is accepted, so the verdict is Unknown, the score is blank and the Checks list says why. NXDOMAIN on every record means the domain does not exist and the address is Undeliverable.
MX, implicit MX and null MX
With MX records present the tool sorts them by priority and resolves the first exchange. If it has an A or AAAA address the domain accepts mail and the Checks list names it. Up to three exchanges are shown in the Details card. If the first exchange does not resolve the address is marked Undeliverable with the hostname that failed.
A domain with no MX but an A or AAAA record falls under the implicit MX rule: senders would try the domain's own address on port 25. Many such domains run no mail server at all, so this is a warning and a 30-point deduction. A single MX of . with priority 0 is a null MX (RFC 7505), an explicit statement that the domain never accepts mail.
Disposable, role and free-provider flags
The domain is compared against a built-in list of temporary-inbox services such as mailinator.com, guerrillamail.com, yopmail.com and 10minutemail.com. A match is a fail and a 60-point deduction, since the inbox will vanish and any account tied to it is effectively anonymous. The list is static, so a new disposable service that is not in it passes.
The local part, with any +tag removed, is compared against role names such as admin, info, support, sales, postmaster, abuse, noreply and billing. A match is a warning worth 15 points because the inbox is shared and often unread. Free consumer providers such as gmail.com or outlook.com are noted as information with no deduction.
Reading the verdict
Undeliverable means the domain cannot receive mail: bad syntax, no such domain, null MX, or an MX that does not resolve. Risky means it can, but something lowers confidence: a disposable domain, or a score under 70 from the deductions above. Deliverable means the domain accepts mail and nothing was flagged beyond free-provider information.
- Unknown: the DNS lookup failed. Retry later or inspect the domain in the DNS lookup
- Did you mean: the suggestion re-runs the check with the corrected domain in the URL
- Mailbox existence isn't confirmed: always shown, because no RCPT TO probe is made
Where to go from here
Results are cached for a few minutes, so a DNS fix can take a moment to show. The address is used for the lookup and nothing else. It is not stored or added to any list. To see whether the domain's mail server actually answers and encrypts, run the host through the SMTP test, which connects to the MX without sending a message. For the complete record set behind a domain, use the DNS lookup with the ALL type.