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How it works

Checking an address means asking the company that runs the mailbox whether it exists, without sending anything to it.

  1. 01

    We read the address

    First the obvious problems: a missing @, a space in the middle, a domain with no ending. If the domain looks like a slip of the finger, such as gmial.com, we tell you the address the person almost certainly meant.

  2. 02

    We find out who handles that email

    Every domain names the company that receives its mail. Some domains name nobody at all, which means they cannot receive email and never could.

  3. 03

    We ask that company about the mailbox

    We open a conversation with the mail server, the same way your email program would, and ask whether it would accept mail for that person. Then we hang up before writing anything. Nothing is delivered, and the person is never told.

  4. 04

    We check the answer is worth anything

    Some companies say yes to every address you ask about, including ones we invent on the spot. We test that by asking about a made-up address at the same domain. If they accept that too, their yes means nothing, and we say so.

  5. 05

    We work out what a no really means

    A refusal can mean the mailbox is gone, or simply that the company will not discuss its addresses with strangers. Those look almost identical, and treating the second as the first is how good customers get deleted. We separate them.

  6. 06

    We tell you how sure we are

    A clear answer from the mail server gets reported plainly. An answer nobody could have obtained is marked uncertain rather than dressed up as a result.

What checking cannot do

These limits apply to every service that does this, ours included. Anyone claiming otherwise is either sending real email or guessing.

Companies that accept anything

Some domains take mail for every possible address so nothing is ever lost. Nobody can confirm one mailbox there without actually sending.

Yahoo, AOL and a few others

These providers deliberately answer yes to everything, to stop people harvesting their addresses. Same outcome, same honest label.

Locked-down company email

Some businesses refuse all questions about their staff addresses as a security measure. Where we have another way to check, we use it. Where we do not, we say the result is unknown.

Real addresses nobody reads

An address can exist, accept mail, and belong to someone who stopped opening it years ago. Checking proves the mailbox is there. It cannot prove anyone is home.

Most addresses on a normal list come back with a clear yes or no. The rest are labelled plainly so you can decide what risk you are comfortable with.

For the technically curious: what the two answers actually look like
Gmail

550 5.1.1 The email account that you tried to reach does not exist.

A statement about the mailbox. Safe to act on.

Microsoft 365

550 5.4.1 Recipient address rejected: Access denied.

A statement about us, not the mailbox. Nothing can be concluded.

Both are refusals, and both carry the same headline code. Only the second part separates "the mailbox is gone" from "I will not answer you".