Email Testing · Mock SMTP Server · Transactional · Staging
Test Transactional Emails Without Sending to Real Users
Zunoy Sandbox is a mock SMTP server and email testing tool for backend developers and QA teams. Every transactional email your app sends in development and staging — welcome emails, password resets, OTPs, order confirmations — lands here instead of a real inbox. Inspect HTML rendering, score deliverability, and validate every detail before a single real user sees it. One SMTP config change. No code changes. No risk.
Works with Node.js · Python · PHP · Laravel · Django · Rails · Any SMTP or REST client
If you're looking for any of these, this page is for you
Zunoy Sandbox is a mock SMTP server and email testing tool — capturing every transactional email your app sends in staging, scoring it automatically, and sharing it with your whole team. Nothing reaches a real user. Everything is inspectable.
Transactional email bugs are invisible until a real user finds them.
Your Staging Emails Are Reaching Real Users
Your developer used a real email address to test the signup flow. The welcome email landed in a real inbox — with a broken link in the CTA, and a subject line that triggered spam filters. The user saw all of it. A mock SMTP server would have caught it before anyone opened their inbox.
You Have No Idea How the Email Actually Looks
Your code says the email sent. Your SMTP logs say 200. But you have never seen what the email looks like in Gmail, Outlook, or on mobile. You're shipping blind and hoping the template renders correctly — which is not email testing, it's hoping.
QA Has No Way to Inspect What Was Sent
QA needs to verify the password reset email contains the right link, the right expiry time, and the right branding. But emails in staging either go to real addresses or get disabled entirely. Neither option gives QA an email testing tool they can actually use to inspect what was sent.
Replace your SMTP provider with a mock SMTP server. Every transactional email lands here instead.
Swap your SMTP config for a mock SMTP server — one environment variable
Replace your real email provider's SMTP host with Zunoy's mock SMTP server in your staging config. Your app sends exactly the same way it always has — same templates, same headers, same logic. The email lands in Sandbox instead of a real inbox. No code changes. No application logic touched. This is what a drop-in email testing tool for backend developers looks like.
Inspect every detail — HTML render, headers, SMTP transaction info
HTML preview across Gmail, Outlook, and Apple Mail. Raw headers. SMTP transaction info including MAIL FROM and RCPT TO. Attachment preview. Every transactional email is fully visible — exactly as your app sent it — the moment it arrives in your shared inbox.
Score every email automatically — HTML, links, spam
HTML Check, Link Check, and Spam Analysis run automatically on every email. Know if your welcome email renders correctly in Outlook 2019, whether your CTA link is live, and whether your subject line triggers SpamAssassin — before it reaches anyone. Your email testing tool that scores without being asked.
Your whole QA team sees it in real time
Every team member sees the same inbox simultaneously. No more "can you forward me that test email?" QA inspects, validates, and signs off — all from the same shared inbox, the moment the email arrives. Staging email testing that the whole team participates in.
Ready to test safely? Start your
free sandbox in under 60 seconds.
One Single Tool for - Email + SMS + Webhook + TOTP
Maintain customer trust - Test Alerts before your Users do
Setup under 5 minutes
Real human support by Zunoy Team
Transactional email testing in staging — configured in under 5 minutes.
Create an inbox and get your mock SMTP server credentials
Create an inbox in Zunoy Sandbox. You get a unique SMTP host, port, username, and password — your mock SMTP server credentials for this environment. Each inbox is isolated with its own credentials. Create one for dev, one for staging, one for QA.
Replace your SMTP config
In your staging environment config, swap your real email provider's SMTP host with your Zunoy mock SMTP server host. Port 8136. SSL required. Username and password from your inbox settings. Everything else — templates, sending logic, headers — stays exactly the same.
Trigger your email flow
Run a test signup, password reset, or order confirmation. Your application fires the email exactly as it would in production — now pointing at your mock SMTP server instead of a real provider.
Email lands in Sandbox instantly
The full email appears in the shared inbox — HTML preview across Gmail, Outlook, Apple Mail, and mobile. Raw headers, SMTP transaction info, attachment preview. Every team member sees it simultaneously.
QA validates, issues fixed, resent
QA checks HTML render, spam score, and link integrity. Issues flagged — developer fixes and resends. New version scores fresh. QA signs off. Transactional email flow ready for production.
Configure your mock SMTP server. One change. That's it.
Replace your real SMTP host with Zunoy's mock SMTP server credentials in your staging environment. Same sending logic. Same templates. Everything else untouched.
These credentials configure Zunoy as your mock SMTP server in staging only. Your production SMTP config — pointing at your real provider — is completely untouched.
const transporter = nodemailer.createTransport({
host: 'mock-smtp.zunoy.com',
port: 8136,
secure: true,
auth: {
user: 'YOUR_INBOX_USERNAME',
pass: 'YOUR_INBOX_PASSWORD',
},
});
await transporter.sendMail({
from: 'app@yourdomain.com',
to: 'customer@example.com',
subject: 'Your order is confirmed',
html: orderConfirmationTemplate,
});Every alternative leaves a gap your users will eventually find.
Real SMTP in staging
Test emails reach real users. Broken templates, staging URLs, and spam-triggering subjects all go live — your email testing happens in production.
Disabling email in staging
You confirm nothing. HTML rendering, broken links, spam score, header validation — all untested. A mock SMTP server costs you nothing. Disabling email costs you visibility.
Single-channel tools like Mailtrap or Mailosaur
Email only. No SMS, no webhook, no TOTP. Each channel needs a separate tool — or one Sandbox account covers all four.
Building an internal mock SMTP server
Engineering overhead. Maintenance cost. No scoring. No team inbox. No spam analysis. Reinventing a wheel that's already built.
Zunoy Sandbox is a mock SMTP server, email inspector, spam checker, and team inbox — all in one. Replace your staging SMTP provider in 60 seconds and start testing transactional emails the way they should be tested.
BUILT FOR
Who uses Sandbox for transactional email testing
Backend developers
Configure Zunoy as your mock SMTP server in staging with one environment variable. Every transactional email your app sends lands in Sandbox — HTML rendered, headers inspectable, scored automatically. Email testing for backend developers that takes 60 seconds to set up.
QA engineers
Inspect every transactional email in a shared inbox — HTML render across Gmail and Outlook, headers, attachments, spam score — without needing a real email address, a real mail client, or a real SMTP provider.
Full-stack developers
Build and test complete transactional email flows — signup, onboarding, password reset, order confirmation — against a mock SMTP server in a safe staging environment before a single real user sees them.
Other testing scenarios covered by Sandbox
SMS OTP testing
Test SMS delivery and OTP codes in staging without sending to a real phone.
OTP / 2FA testing
Generate and verify TOTP codes in staging without real authenticator apps.
Webhook testing
Inspect, replay, and debug incoming webhook payloads with a persistent request log.
The things developers ask about Transactional Email Testing
What is a mock SMTP server and how does Zunoy work as one?
A mock SMTP server is a replacement SMTP endpoint that accepts email sending requests without forwarding them to a real provider or a real inbox. Zunoy acts as a mock SMTP server in your staging environment — receiving every transactional email your app sends, capturing the full payload including HTML, headers, and SMTP transaction info, and making it instantly available to your team. Configure it with one environment variable change. No code changes. No real emails sent.
How do I test transactional emails without sending to real users?
Replace your real SMTP provider's host with Zunoy's mock SMTP server in your staging environment config. Your app sends the same way it always has — the email lands in Sandbox instead of a real inbox. Full HTML preview, headers, spam score, and link check available immediately. Nothing reaches a real user unless you explicitly configure forwarding to a pre-verified address.
Is Zunoy a good email testing tool for backend developers?
Yes. Zunoy Sandbox is designed specifically as an email testing tool for backend developers — SMTP and REST API connection, per-inbox isolation with unique credentials, HTML rendering across Gmail and Outlook, automatic spam scoring, and a shared team inbox. Configure it in under 5 minutes. No application code changes required.
Is email scoring automatic on every transactional email?
HTML Check, Link Check, and Spam Analysis run automatically on every email that arrives — no trigger needed. Performance Score and AI Score are available on demand for deeper analysis. See Email Scoring for full details on what each dimension checks.
Can my QA team inspect transactional emails without a separate account?
Yes. Email Inbox is shared in real time across all team members on your Sandbox account. Everyone sees incoming emails simultaneously — HTML preview, headers, scores — no refresh required, no separate login needed.
Every Testing Scenario, One Sandbox Workspace
Your app sends emails, SMS, webhooks, and TOTP codes. Sandbox handles all four — in one workspace, under one account, visible to your whole team.
Email Inbox
SMS Inbox
Webhook Inbox
TOTP Inbox
Sandbox
Test Email, SMS, Webhooks & 2FA codes without touching production.
Operational
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