Mailsac Alternative in 2026: Developer-First Email Testing with Real OTP Extraction

Introduction

Mailsac has served developers well for over a decade. Its zero-setup public inbox model — send to any @mailsac.com address and read it immediately — is genuinely elegant. WebSocket support, REST API, and webhook routing give it a solid technical foundation.

But Mailsac's architecture has trade-offs that matter in production QA environments. Public inboxes mean anyone can read your test emails. There is no built-in OTP extraction. The ops-based pricing model requires understanding which endpoints cost which ops. Billing is yearly-only on most plans. And there is no SDK beyond a community Node.js library and no MCP interface for AI workflows.

FreeCustom.Email solves all of these in one package — official JavaScript and Python SDKs, built-in OTP extraction, private inboxes by default, long-polling, WebSocket push, and monthly billing. This guide covers the full comparison.

What Mailsac Does Well

• Zero-setup receiving: Any @mailsac.com address works immediately with no inbox registration

• WebSocket support: Real-time message delivery via WebSocket — ahead of many competitors

• REST API: Well-documented with reasonable endpoint coverage

• Enterprise maturity: 10+ years, SAML SSO, purchase-order billing, VRA support

• Email validation: Built-in email address format and disposability checking

Where Mailsac Falls Short

Pricing Comparison

Installation

JavaScript / TypeScript

npm install freecustom-email

Python

pip install freecustom-email

CLI

npm install -g fcemail

SDK vs Raw WebSocket: Replacing Mailsac's WebSocket

Mailsac WebSocket (current usage — community library, manual OTP parsing)

const WebSocket = require('ws');
const ws = new WebSocket(
  'wss://mailsac.com/api/emails/test@mailsac.com/ws?_mailsacKey=YOUR_KEY'
);

ws.on('message', data => {
  const email = JSON.parse(data);
  // Manual OTP extraction — your regex
  const match = email.text?.match(/\b(\d{6})\b/);
  const otp = match?.[1];
  console.log('OTP:', otp);
});

FreeCustom.Email SDK WebSocket (OTP pre-extracted, auto-reconnect)

import { FreecustomEmailClient } from 'freecustom-email';

const client = new FreecustomEmailClient({
  apiKey: process.env.FCE_API_KEY!,
});

// Register inbox first
await client.inboxes.register('test@ditapi.info');

const ws = client.realtime({
  mailbox: 'test@ditapi.info',
  autoReconnect: true,
  reconnectDelayMs: 3_000,
  maxReconnectAttempts: 10,
  pingIntervalMs: 30_000,
});

ws.on('connected', info => {
  console.log('Plan:', info.plan);
  console.log('Subscribed:', info.subscribed_inboxes);
});

ws.on('email', email => {
  // OTP and link already extracted — no regex needed (Growth+)
  console.log('OTP:', email.otp);
  console.log('Link:', email.verificationLink);
  console.log('From:', email.from);
});

ws.on('reconnecting', ({ attempt, maxAttempts }) =>
  console.log(`Reconnecting ${attempt}/${maxAttempts}...`)
);

ws.on('error', err => {
  if (err.upgrade_url) console.log('Upgrade at:', err.upgrade_url);
});

await ws.connect();
// ws.disconnect() when done

# Python SDK — WebSocket
from freecustom_email import FreeCustomEmail

client = FreeCustomEmail(api_key="fce_...")

await client.inboxes.register("test@ditapi.info")

ws = client.realtime(
    mailbox="test@ditapi.info",
    auto_reconnect=True,
    reconnect_delay=3.0,
    max_reconnect_attempts=10,
    ping_interval=30.0,
)

@ws.on("connected")
async def on_connect(info):
    print(f"Plan: {info.plan}")

@ws.on("email")
async def on_email(email):
    print(f"OTP: {email.otp}")           # pre-extracted (Growth+)
    print(f"Link: {email.verification_link}")

@ws.on("error")
async def on_error(event):
    if event.upgrade_url:
        print(f"Upgrade at: {event.upgrade_url}")

await ws.connect()
await ws.wait()

See WebSocket documentation.

Replacing Mailsac Webhooks

Mailsac supports webhooks for private addresses. FreeCustom.Email's webhooks include pre-extracted OTP and verification link — no server-side parsing.

// JavaScript SDK — register webhook
const hook = await client.webhooks.register(
  'test@ditapi.info',
  'https://your-server.com/callback',
);
console.log('Webhook ID:', hook.id);

// List webhooks
const hooks = await client.webhooks.list();

// Unregister
await client.webhooks.unregister(hook.id);

# Python SDK
hook = await client.webhooks.register(
    inbox="test@ditapi.info",
    url="https://your-server.com/callback",
)
print(f"Webhook ID: {hook.id}")
await client.webhooks.unregister(hook.id)

Webhook payload:

{
  "event": "new_message",
  "inbox": "test@ditapi.info",
  "message": {
    "id": "msg_01jqz3k4m5n6p7q8",
    "from": "noreply@yourapp.com",
    "subject": "Your verification code",
    "otp": "482931",
    "verification_link": "https://yourapp.com/verify?token=abc123"
  }
}

Verify webhook signature (HMAC-SHA256):

import crypto from 'crypto';

function verifyWebhook(rawBody: string, signature: string, apiKey: string): boolean {
  const expected = crypto
    .createHmac('sha256', apiKey)
    .update(rawBody)
    .digest('hex');
  return crypto.timingSafeEqual(
    Buffer.from(expected, 'hex'),
    Buffer.from(signature, 'hex'),
  );
}

import hmac, hashlib

def verify_webhook(raw_body: bytes, signature: str, api_key: str) -> bool:
    expected = hmac.new(
        api_key.encode(), raw_body, hashlib.sha256
    ).hexdigest()
    return hmac.compare_digest(expected, signature)

See webhooks documentation.

The Privacy Problem: Public vs Private Inboxes

Mailsac's core model is public inboxes. Send to test@mailsac.com and anyone can read it. This creates two QA problems:

1. Security: Sensitive test tokens, passwords, and PII in test emails are publicly exposed.

2. Isolation: Another process can consume your email before your test does — causing random failures.

FreeCustom.Email inboxes are private by default. Only your API key can read them.

// Register a private inbox — no one else can read it
await client.inboxes.register('qa-user-123@ditapi.info');

// List your registered inboxes
const inboxes = await client.inboxes.list();
console.log(inboxes); // [{ inbox: 'qa-user-123@ditapi.info', local: 'qa-user-123', domain: 'ditapi.info' }]

// Unregister when done
await client.inboxes.unregister('qa-user-123@ditapi.info');

See inbox management documentation.

Long-Polling: What Mailsac Lacks

Mailsac has no long-poll endpoint. With FreeCustom.Email's SDK, the messages.waitFor() method uses the server-side /wait endpoint — the connection stays open until an email arrives, or the timeout is hit:

// JavaScript — server-side long-poll, no client loop
const msg = await client.messages.waitFor('test@ditapi.info', {
  timeoutMs: 30_000,
  pollIntervalMs: 2_000,
  match: m => m.subject.includes('verify'),
});
console.log('OTP:', msg.otp);
console.log('Link:', msg.verificationLink);

# Python
msg = await client.messages.wait_for(
    "test@ditapi.info",
    timeout_ms=30_000,
    poll_interval_ms=2_000,
    match=lambda m: "verify" in m.subject,
)
print(f"OTP: {msg.otp}")

See Wait API documentation.

Full OTP Flow (One Method Call)

// JavaScript
const otp = await client.getOtpForInbox(
  'test@ditapi.info',
  async () => {
    await fetch('https://yourapp.com/api/signup', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ email: 'test@ditapi.info' }),
    });
  },
  { timeoutMs: 30_000, autoUnregister: true },
);
console.log('OTP:', otp);

# Python
import httpx

async def trigger():
    async with httpx.AsyncClient() as http:
        await http.post(
            "https://yourapp.com/api/signup",
            json={"email": "test@ditapi.info"},
        )

otp = await client.get_otp_for_inbox(
    inbox="test@ditapi.info",
    trigger_fn=trigger,
    timeout_ms=30_000,
    auto_unregister=True,
)
print(f"OTP: {otp}")

CLI: Quick Terminal Workflows

# Authenticate once
fce auth login

# Create inbox
fce inbox create --domain ditapi.info

# Real-time email stream in terminal
fce watch test@ditapi.info

# Extract OTP for shell scripting
OTP=$(fce otp test@ditapi.info)
echo "OTP: $OTP"

See CLI documentation.

MCP for AI Agents (Growth+ plans)

Mailsac has no MCP interface. FreeCustom.Email's MCP server enables AI agents to handle email verification autonomously:

{
  "mcpServers": {
    "fce-mcp": {
      "command": "npx",
      "args": ["-y", "fce-mcp-server"],
      "env": { "FCE_API_KEY": "your_growth_key" }
    }
  }
}

See MCP documentation and AI agent use case.

Custom Domains

// JavaScript SDK — add and verify a custom domain
const result = await client.domains.addCustom('mail.yourdomain.com');
console.log('DNS records to add:', result.dns_records);
// [{ type: 'MX', hostname: '@', value: 'mx.freecustom.email', priority: '10' },
//  { type: 'TXT', hostname: '@', value: 'freecustomemail-verification=...' }]

const v = await client.domains.verifyCustom('mail.yourdomain.com');
console.log('Verified:', v.verified);

# Python SDK
result = await client.domains.add_custom("mail.yourdomain.com")
for rec in result.dns_records:
    print(f"{rec.type} {rec.hostname} → {rec.value}")

v = await client.domains.verify_custom("mail.yourdomain.com")
print(f"Verified: {v.verified}")

See custom domains documentation.

Feature Comparison

FAQ

Q: Does FreeCustom.Email require inbox registration before receiving mail? Yes — one API call or SDK call per inbox. This ensures privacy isolation. Mailsac's zero-setup model is convenient but comes at the cost of public exposure.

Q: Can I use FreeCustom.Email's WebSocket with my existing Mailsac WebSocket code? The SDK abstracts the connection layer. Migrate by replacing the Mailsac ws constructor with client.realtime(). The event names differ — see WebSocket documentation.

Q: Does FreeCustom.Email support SAML SSO? Not currently. If SAML SSO is a hard enterprise requirement, Mailsac's Business plan covers that.

Q: What platform domains are available? See platform domains documentation for the current list.

Q: Is there a Mailsac-like email validation feature? FreeCustom.Email focuses on inbox delivery and testing rather than email validation. For validation, Mailsac's validation API remains useful.

Conclusion

Mailsac pioneered reliable disposable email for developers. But public inboxes by default, no OTP extraction, no official SDK, and yearly-only billing make it a difficult fit for modern QA pipelines that require isolation, speed, and automation.

FreeCustom.Email delivers private inboxes, official JavaScript and Python SDKs with full TypeScript types, automatic OTP extraction, and monthly billing that scales with your actual usage.

→ Get started free → npm install freecustom-email → pip install freecustom-email → Compare pricing plans → Try the API playground