Test SMTP credentials without trusting anyone

Name: SMTP Login Checker
Author: dotreflection

Run a real SMTP authentication handshake against any mail server and see exactly what happens — step by step. Everything runs on this machine. Your username and password are sent only to the server you name, never to dotreflection or any other third party.

Launch it on your machine — one command, nothing installed permanently:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/dotreflection/smtp-login-checker/main/install.sh)"

Downloads the source into a temporary folder, runs it locally, and opens this page in your browser. Requires Node.js 18+. The folder is deleted when you stop the tool with Ctrl+C.

Stays on your machine The server is bound to 127.0.0.1. Credentials go straight to your mail server.
Open source Every line is auditable on GitHub. No hidden network calls.
Free, no account A free tool from dotreflection. Nothing to sign up for, nothing logged or stored.

This is a preview — the form below is disabled here. For your security, credential tests run on your own machine, never on this site. Run the command above to start the local checker; it opens this page on 127.0.0.1 with the form fully working.

SMTP host

Port

Encryption

Username

Password

Send test email to optional

Add a recipient and the button below switches to “Send test email”. Leave it empty to just check the login.

Advanced options

Auth mechanism

Timeout (seconds)

From address

Verify TLS certificate

Uncheck only for servers with self-signed certificates you trust.

Authenticates and disconnects. With a recipient above, it also delivers one short test message.

Result

Session transcript

Credential bytes are redacted below. The raw values never appear in this log.

Why does this run locally?

A web page in a browser cannot open a raw SMTP connection — browsers only speak HTTP and WebSockets. So any “SMTP tester” that lives purely on a website has to forward your password to its own backend to do the actual login. That means trusting a stranger with working mail credentials.

This tool avoids that entirely. You run a tiny program on your own computer; it serves this page and performs the SMTP handshake itself. The only outbound connection ever made is from your machine directly to the mail server you typed in. dotreflection never sees anything.