BranchesTags

Repository files navigation

mail-check

mail-check

Verify if email addresses actually exist — without sending them.
mail-check.t21.dev

Features

  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)

Live

https://mail-check.t21.dev

CLI

npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output

How It Works

mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.

Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:

  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers

SMTP & Port 25

SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.

For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.

Self-Hosting

Run Locally

The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.

git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev

Open http://localhost:3000. SMTP checks will work out of the box.

Deploy on a VPS

For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.

git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start

Or use Docker:

docker build -t mail-check .
docker run -p 3000:3000 mail-check

Cloud Platforms (limited)

You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.

Tech Stack

  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono

Project Structure

app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API

POST https://mail-check.t21.dev/api/check

{ "emails": ["test@gmail.com"] }

Returns results with reachability status: safe, risky, invalid, or unknown.

License

MIT

About

Check if email addresses exist — without sending them.

mail-check.t21.dev

Topics

emailemail-marketingemail-senderemail-validationsmtp

Resources

Readme
Activity
Custom properties

Stars

0 stars

Watchers

0 watching

Forks

0 forks
Report repository

Releases

Packages

Contributors

Languages

, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

Skip to content
BranchesTags
Repository files navigation
mail-check
mail-check
Verify if email addresses actually exist — without sending them.
mail-check.t21.dev
Features
  • Single and bulk email validation (up to 100 per request)
  • MX record lookup + SMTP mailbox probing
  • Disposable & catch-all email detection
  • IP-block aware (detects Spamhaus/blacklist rejections)
  • CSV import / export for bulk results
  • PWA with offline detection
  • IP-based rate limiting (100 req / 10 min)
Live
https://mail-check.t21.dev
CLI
npx tsx cli.js user@gmail.com # single check
npx tsx cli.js a@test.com b@test.com # multiple
npx tsx cli.js --csv sample.csv # CSV file
npx tsx cli.js --json user@gmail.com # JSON output
How It Works
mail-check connects directly to mail servers via SMTP (port 25) to verify if a mailbox exists — without actually sending an email.
Your server
│
│ DNS lookup → MX records
▼
Gmail / Outlook / Yahoo MX servers
│
│ SMTP handshake (EHLO → MAIL FROM → RCPT TO)
▼
250 OK = exists, 550 = doesn't exist

The app performs these checks in order:
  1. Syntax — validates email format
  2. MX lookup — finds the domain's mail servers
  3. SMTP probe — connects to the mail server and checks if the mailbox accepts mail
  4. Flags — detects catch-all domains and disposable email providers
SMTP & Port 25
SMTP verification requires outbound port 25. Most cloud platforms (Railway, Vercel, Render, AWS, etc.) block this port to prevent spam abuse. The hosted version at mail-check.t21.dev may show limited SMTP results due to this restriction.
For full accuracy, run mail-check locally or self-host on a VPS where port 25 is open.
Self-Hosting
Run Locally
The easiest way to get full SMTP verification — most home/office ISPs don't block port 25.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run dev
Open http://localhost:3000. SMTP checks will work out of the box.
Deploy on a VPS
For a persistent setup, deploy on any VPS with port 25 open. Providers like Hetzner, DigitalOcean, or BuyVM ($2–5/mo) work well.
git clone https://github.com/t21dev/mail-check.git
cd mail-check
npm install
npm run build
npm start
Or use Docker:
docker build -t mail-check .
docker run -p 3000:3000 mail-check
Cloud Platforms (limited)
You can deploy to Railway, Vercel, Netlify, etc., but SMTP verification will be unavailable since these platforms block port 25. The app gracefully falls back to syntax + MX + disposable detection only — SMTP results will show as unknown.
Tech Stack
  • Framework: Next.js 16 (App Router) + React 19 + TypeScript
  • Styling: Tailwind CSS v4 + shadcn/ui
  • Fonts: Outfit + JetBrains Mono
Project Structure
app/
layout.tsx Root layout
page.tsx Home page
api/check/route.ts POST endpoint for email checking
server/
services/ Core logic (email-checker, rate-limiter)
actions/ Server Action wrappers
components/ SingleCheck, BulkCheck, ResultBadge, ThemeToggle
types/ Shared TypeScript interfaces
cli.js CLI entry point

API
POST https://mail-check.t21.dev/api/check
{ "emails": ["test@gmail.com"] }
Returns results with reachability status: safe, risky, invalid, or unknown.
License
MIT
About
Check if email addresses exist — without sending them.
mail-check.t21.dev
Topics
emailemail-marketingemail-senderemail-validationsmtp
Resources
Readme
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
0 forks
Report repository
Releases
Packages
Contributors
Languages