Both tools show up under “email testing” searches, but they solve opposite halves of the problem. This walkthrough covers what each is actually for, where they overlap, and how to pick for your use case.
TL;DR
| Mailtrap | MailSink | |
|---|---|---|
| What it does | Catches outbound email before it ships | Provisions real inboxes that receive mail |
| API shape | SMTP sandbox + REST | REST + MCP |
| Intended user | Devs staging new transactional templates | QA pipelines and AI agents |
| Retention | Long, volume-based | TTL-based (1 hr free, 7 days Team) |
| MCP support | No | Yes, via @mailsink/mcp |
| Starting price | Free (100 emails/mo) | Free (50 inboxes/mo, MCP included) |
| Best at | Staging outbound email safely | Automated signup + OTP extraction |
| Worst at | CI signup verification, agent inboxes | Previewing outbound template safety |
If you’re standing up a SaaS and want to see how a welcome email looks in Outlook, Mailtrap. If you want a Playwright test that signs up, reads the verification code, and asserts, MailSink.
What Mailtrap actually does
Mailtrap is a sandbox for outbound email. Your app’s SMTP or API points at Mailtrap instead of a real provider. Anything your app sends lands in a shared inbox visible in Mailtrap’s UI and never reaches a recipient.
Sweet spots:
- Staging templates. A dev working on a new transactional email sees the rendered HTML without spamming real users.
- Spam-score previews. Mailtrap runs templates through SpamAssassin and flags issues.
- Cross-client render tests. Screenshots in Outlook, Gmail, Apple Mail.
- Production delivery. Mailtrap also sells a sending product for real outbound, but that’s a separate service with its own pricing.
What Mailtrap is not: it does not give your tests a real inbox address. A Playwright test that submits [email protected] to a signup form gets nothing because that address does not exist on a real MX. Mailtrap only captures mail that your own app sent through its SMTP.
What MailSink actually does
MailSink provisions real, disposable email addresses. An address like [email protected] is live on a real MX. Stripe can send to it. So can GitHub, Supabase, your own staging server, or a human.
Sweet spots:
- CI email verification. Your test suite creates an inbox, submits the address to your app, waits for real mail to arrive, asserts on the OTP or verification link.
- AI agents. Agents use the MCP server to provision inboxes and read verification codes without touching a human’s mail.
- Third-party signup testing. Test onboarding flows when the provider is the sender (Stripe, Auth0, Clerk).
What MailSink is not: it does not catch outbound email from your app. If you are sending real mail and want a preview sandbox, you want Mailtrap.
Feature-by-feature
API shape
Mailtrap exposes SMTP (the capture endpoint) plus REST for managing sandboxes and messages. Fine for libraries that already speak SMTP.
MailSink is REST-only. Three endpoints cover most workflows:
# Create an inbox
curl -X POST https://api.mailsink.dev/v1/inboxes \
-H "Authorization: Bearer msk_..."
# → { "id": "inb_rae1z", "address": "[email protected]", ... }
# Wait for the verification code (long-poll, up to 60s)
curl https://api.mailsink.dev/v1/inboxes/inb_rae1z/wait-for-code \
-H "Authorization: Bearer msk_..."
# → { "code": "847291", "from": "[email protected]", ... }MailSink also ships an MCP server so Claude Desktop, Claude Code, and Cursor agents can drive inboxes directly. Mailtrap has no MCP equivalent as of April 2026.
Retention
Mailtrap holds captured email per your plan’s message limit.
MailSink uses TTL. Free: 1 hour. Pro: 24 hours. Team: 7 days. After TTL, the message is deleted from both D1 metadata and R2 object storage. If your test finishes in 30 seconds, a 1-hour TTL is more than enough and keeps your account clean automatically.
Pricing
Mailtrap Testing starts free (100 emails/mo) and climbs through $9.99, $14.99, $24.99, up to $129.99+ by test email volume. The separate Email Delivery product has its own ladder.
MailSink:
- Free: 50 inboxes/month, MCP access, 60 req/min
- Pro: $15/mo, 2,000 inboxes, 600 req/min, 24h TTL
- Team: $49/mo, 20,000 inboxes, 3,000 req/min, 7-day TTL, 5 MB email size
No per-email pricing; you pay for inbox provisioning. This works well for CI runs where a single inbox receives a few messages per test.
CI integration
Both tools plug into CI. Mailtrap’s SMTP target works with any library that can speak SMTP, so nodemailer, go-mail, smtplib, and friends all work without special logic.
MailSink integrates via HTTP. There are worked examples for Playwright and Cypress, plus a drop-in GitHub Actions pattern for running full signup tests inside a pipeline.
AI agents
Mailtrap has no agent story. You could script against its REST API, but each agent would need its own SMTP client and no tool call exists for “give me the latest verification code.”
MailSink ships @mailsink/mcp with eight typed tools: create_inbox, wait_for_email, get_verification_code, get_verification_link, list_messages, list_inboxes, get_message, delete_inbox. Any MCP client picks them up. The MCP post has full configs for Claude Desktop, Claude Code, and Cursor.
When to pick Mailtrap
- You’re building a product that sends transactional email and you need a staging sandbox for templates.
- You care about spam-score previewing or render tests across mail clients.
- Your test is “did my app send the right email?”, not “did my app accept the OTP a third party sent?”
When to pick MailSink
- You want CI to verify the full signup flow end to end, including real verification email from a third party.
- You’re building an AI agent that needs to sign up for services and catch OTPs.
- You want throwaway inboxes that auto-clean, not a shared sandbox that accumulates.
- You want an MCP-first story for automation.
Can you use both?
Yes. They occupy different layers. A mature test setup might use Mailtrap to assert on outbound welcome templates (what your app sends) and MailSink to verify the incoming signup flow (what third parties send you). They don’t conflict.
FAQ
Does Mailtrap have an MCP server?
Not as of April 2026. MailSink’s package is @mailsink/mcp on npm and works with Claude Desktop, Claude Code, Cursor, and any other MCP client.
Can MailSink preview what my outbound email looks like?
No. Use Mailtrap for that. MailSink only receives mail at addresses it provisions for you.
Can I use MailSink on a domain I already own?
Bring-your-own-domain support is on the roadmap for Pro and Team. Join the waitlist if this is important for your team.
Are MailSink inboxes really random?
Yes. Addresses follow {tag}-{6-char-random}@{shared-domain}. Collision is astronomically unlikely, and every address is account-scoped so no two accounts share one.
What about Mailslurp, Mailosaur, Mailinator?
Different trade-offs across all three. They’re closer to MailSink’s shape (real disposable inboxes) than to Mailtrap’s. Comparison posts on those are coming.
Next
- Try MailSink free (50 inboxes/month, no credit card)
- MCP server guide for agents
- API reference
- Mailtrap pricing page for their latest plans