> For the complete documentation index, see [llms.txt](https://docs.mailtrap.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mailtrap.io/email-sandbox/testing/bounce-rate.md).

# Bounce Emulator

Learn how to test your SMTP client's behavior when the server responds with an error

## Constructing the address

The recipient's username should start with bounce+ and contain the response in URL-encoded form: `bounce+550+no+such+user+here@inbox.mailtrap.io`. The domain must be `inbox.mailtrap.io`.

If you send to `bounce@inbox.mailtrap.io` without a custom response, you'll get the default one: `550 5.1.1 The email account that you tried to reach does not exist.`

To create it, use [https://www.urlencoder.org](https://www.urlencoder.org/) or any other URL encoding solution.

Tip: Write the address in lowercase. For example, `Bounce+...` won't trigger the emulator. To get capital letters in the response text, use URL encoding:  `bounce+550+%4Eo+such+user+here@inbox.mailtrap.io` returns `550 No such user here`.

## Using Bounce Emulator with an email client

Just use the inbox.mailtrap.io host with any email client or application and send an email to `bounce+451+server+unavailable@inbox.mailtrap.io` .

## Using Bounce Emulator with Sandbox

If your application is connected to Email Sandbox SMTP, send an email from the application to `bounce+454+authentication+required@inbox.mailtrap.io` , and your application will receive a bounce response from the Sandbox SMTP server. Bounce addresses on any other domain are captured in your inbox as regular emails.

*Note: This feature does not work with API, as bounce codes are specific to SMTP.*


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# Agent Instructions
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## Querying This Documentation
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```
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```

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