Turn on real mailbox checks.
By default MailProof checks the domain. To confirm whether each individual mailbox exists, run one small helper on your computer. It’s free, open source, takes a minute, and nothing you check ever leaves your machine.
Why a helper?
Confirming a mailbox means asking its mail server, over a connection called SMTP, “does this address exist?” — without sending anything. Browsers and cloud servers aren’t allowed to open that kind of connection, so the check has to run on a normal computer. That’s all the helper is: a tiny program that does the one thing the browser can’t, then answers the MailProof tab on your own machine.
Step 1 — Install Node.js
The helper runs on Node.js. If you don’t already have it, download the “LTS” version from nodejs.org and install it (Next, Next, Done). To check whether it’s already there, open a terminal and run:
node --versionAny version number means you’re set.
Step 2 — Start it, in one command
Paste this whole line into Terminal and press Enter. It downloads the helper into your Downloads folder and starts it, so there is no folder to get wrong:
cd ~/Downloads && curl -fO https://email-verifier-red.vercel.app/mailproof-helper.js && node mailproof-helper.jsYou’ll see:
MailProof helper is running.
Real mailbox verification is now ON.Leave that window open. Come back to this site (or the extension) — the badge flips to Real mailbox check on its own, and results now say Deliverable or Undeliverable for the actual mailbox. Done with checking? Press Ctrl+C in that window to stop it.
Prefer not to use Terminal at all? Download the file, then double-click it from Finder is not enough on its own — macOS needs it started by Node, so the one-liner above is the reliable route. The file itself is here if you want to read it first:
If something goes wrong
- “command not found: node” — Node isn’t installed. Get the LTS build from nodejs.org and run the command again.
- “Port 8025 is already in use” — the helper is already running in another window. You’re done; nothing else to do.
- The badge stays on “Domain checks” — make sure the Terminal window is still open, then hit Recheck in the extension.
What you’ll get
- Deliverable — the mail server confirmed the mailbox exists.
- Undeliverable — the server rejected it, or the domain is dead. Safe to remove.
- Domain OK — the domain takes mail but this one couldn’t be confirmed (see the honest limits below).
Honest limits
This is real verification, not magic. Two situations genuinely can’t be resolved by anyone, and MailProof says so rather than guessing:
- Catch-all domains. Some companies configure their mail server to accept every address and sort it out internally. There’s no way to confirm a single mailbox on those, so they come back as Domain OK.
- Rate limits. Large providers may slow down or temporarily block repeated checks from one computer. If you’re verifying a big list, expect it to take a while, and check in reasonable batches. Gmail and most providers work well for everyday volumes.
- Port 25. A few home ISPs block the port SMTP uses. If every result comes back “Domain OK”, that’s the likely cause — try a different network.
Is it safe?
Yes. The helper only makes the DNS and SMTP lookups a verification needs, and only ever talks to your own browser tab over localhost. It stores nothing, sends no email, and opens no ports to the outside world. The code is a single readable file — open it and see.