How I Built a Free Inbox Placement Test with Node.js and SSE

The Problem

I've been doing B2B cold outreach for 5+ years. The #1 blind spot? ESPs confirm SMTP delivery — status 250, "message accepted." But they never tell you which folder your email actually landed in.

Your campaign could have a 0% open rate not because of bad copy, but because every email is sitting in Spam. And you'd never know.

The Solution

I built a free tool that answers one question: did my email hit Inbox, Spam, or Promotions?

Try it here: https://check.live-direct-marketing.online/

How It Works

You enter your email address
You get 5-7 seed addresses across Gmail, Outlook, Yahoo, Mail.ru, and Yandex
You send your test email from your real mailing system
Results stream in live — placement per provider + full authentication report

Tech Stack

Backend: Node.js
Live updates: Server-Sent Events (SSE) — lightweight, no WebSocket overhead
Folder detection: IMAP connection to seed mailboxes, parsing folder placement
Authentication parsing: SPF, DKIM, DMARC results extracted from email headers
Frontend: Vanilla JS, no framework — keeps it fast

Why SSE over WebSocket?

For this use case SSE is a better fit. The data flows one direction: server → client. No need for bidirectional communication. SSE auto-reconnects, works through proxies, and is dead simple to implement:

const evtSource = new EventSource('/api/test/stream?id=' + testId);
evtSource.onmessage = (e) => {
  const result = JSON.parse(e.data);
  updateUI(result);
};

IMAP Folder Detection

The core logic connects to each seed mailbox via IMAP, searches for the test email by Message-ID, and checks which folder it landed in:

INBOX → your email is fine
[Gmail]/Spam, Junk, etc. → problem with reputation or authentication
[Gmail]/Promotions → not critical, but reduces visibility

Authentication Breakdown

Every incoming email has authentication headers. The tool parses them and gives you a clear SPF/DKIM/DMARC status — pass, fail, or missing. This is usually the first thing to fix if you're landing in Spam.

What I Learned

Mail.ru and Yandex matter if you target CIS markets. Most tools ignore them.
SPF/DKIM/DMARC issues are the #1 reason emails land in Spam — not content, not subject lines.
Live results (SSE) make a huge UX difference vs "check back in 5 minutes."