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mailboxsimulator

SMTP testing for developers

Test your email flow.
Delivered. Deferred. Bounced.

Simulate successful delivery, bounces, and deferrals with a test email. Check how your app handles each response before it affects a customer.

Choose a test address

Seven addresses. Predictable SMTP responses.

Choose the outcome you want to test

Each address returns a specific SMTP response. Copy one to start your test.

  • Delivered

    Accepted and discarded.

    success@mailboxsimulator.com

    2502.0.0

  • Hard bounce

    A permanent delivery rejection.

    hardbounce@mailboxsimulator.com

    5505.0.0

  • Soft bounce

    A temporary delivery rejection.

    softbounce@mailboxsimulator.com

    4504.0.0

  • Deferred

    A temporarily unavailable mailbox.

    deferred@mailboxsimulator.com

    4504.2.0

  • Mailbox full

    A full mailbox that cannot accept mail.

    mailboxfull@mailboxsimulator.com

    4504.2.2

  • User unknown

    A nonexistent recipient address.

    userunknown@mailboxsimulator.com

    5505.1.1

  • Blocked

    A policy rejection, not a spam scan.

    blocked@mailboxsimulator.com

    5505.7.1

250 means accepted, 450 means a temporary failure, and 550 means a permanent failure. The second code gives more detail about the outcome.

The success address returns 250 at the recipient step (RCPT TO) and 250 2.0.0 after the message body (DATA). All other scenarios reject at the recipient step.

From test address to SMTP response

  1. 01 / Choose

    Copy a test address

    Choose a scenario above. Start with successful delivery, then test how your app handles failures.

  2. 02 / Send

    Send through your app

    Use the address as the recipient in your existing email flow. The subject and body don’t change the outcome.

  3. 03 / Inspect

    Verify your handling

    Check your sender’s logs or provider dashboard. Confirm that your app handles acceptance, retries, and permanent failures as expected.

Label each test run

Add + and a label before the @ in any test address. Use an order ID or test name to find the recipient in your own logs; the response stays the same.

hardbounce+order123@mailboxsimulator.com

Know what to expect

Successful delivery is simulated: the server accepts and discards the message. There is no inbox to check, and the simulator sends no bounce emails or event reports.

Temporary failures return the same response on every retry; they never switch to success automatically. Your sending provider decides when to retry and how to report the result.