Verifying an email address without sending to it

There is no registry of mailboxes. The only authority on whether jane@example.co.uk exists is example.co.uk's own mail server, and it is under no obligation to tell you. Verification is therefore a series of narrowing checks, each cheap and each only partly conclusive.

The checks, in order

1. Syntax. The address has to obey the shape in RFC 5321 and 5322: a mailbox name of at most 64 characters without leading, trailing or doubled dots, an @, and a real domain. Most "invalid" addresses on sign-up forms fail here, usually through a typo.
2. Domain and MX. The domain must exist and publish MX records (or, as a fallback, an A record for a host that runs SMTP). A null MX record, a single MX of ".", is a deliberate statement that the domain receives nothing.
3. Classification. Disposable providers hand out mailboxes that live for minutes; role addresses such as info@ or support@ are shared and often filtered from lists; free webmail is fine for people but tells you a business has no domain of its own.
4. The SMTP conversation. Connect to the lowest-preference MX, say EHLO, give a sender, and issue RCPT TO:<jane@example.co.uk>. A 250 means the server will accept mail for that name; a 550 with "unknown user" or "no such recipient" means it will not. Then quit without sending anything.

Why the answer is often "unknown"

• Catch-all domains accept every name at RCPT time and sort it out later, so a 250 proves nothing. The checker sends a second RCPT for a random name; if that is accepted too, the domain is a catch-all and the verdict is "risky" rather than "deliverable".
• Greylisting answers the first attempt from a new sender with a temporary 4xx and expects a retry minutes later. Real mail gets through; a probe cannot wait.
• Sender verification means the receiving server first checks that the sender address exists before answering about the recipient. The checker uses a real mailbox for exactly this reason.
• Large providers vary. Gmail answers honestly at RCPT time; Microsoft 365 tenants may accept everything and bounce later; some corporate gateways refuse to confirm anything.

Reading the verdicts

Deliverable means the server accepted the mailbox and refused a random one. Probably deliverable means the domain accepts mail but the mailbox was not probed. Risky covers catch-all domains, disposable providers and domains with no MX. Unknown means the server would not say. Undeliverable means the mailbox was rejected, or the domain cannot receive mail at all.

Etiquette

Verification probes are a small load on someone else's server. whois.me.uk runs the SMTP step only for signed-in accounts, never sends message content, and never stores the address.