A test email that succeeds tells you the happy path works. But what happens when a recipient address does not exist, a mailbox is full, or delivery is temporarily deferred?
We built Mailbox Simulator, a free SMTP testing tool by Prontuno, to help developers answer that before a customer is affected. Send an email from your application to one of its test addresses. Each address returns a predictable SMTP response, so you can check what your sending provider reports and how your application handles the outcome.
Choose the outcome you want to test
Mailbox Simulator provides seven addresses covering common delivery outcomes:
- Delivered: The server accepts the message with a
250 2.0.0response. - Hard bounce: A permanent
550 5.0.0rejection. - Soft bounce: A temporary
450 4.0.0rejection with no specific reason. Use it to test your general retry handling. - Deferred: A temporary
450 4.2.0response indicating the recipient’s mailbox is temporarily unavailable. Use it to test how your application reports messages that are delayed rather than failed. - Mailbox full: A temporary
450 4.2.2response. - User unknown: A
550 5.1.1response for a nonexistent recipient. - Blocked: A
550 5.7.1policy rejection.
Rejections happen at the recipient step (RCPT TO), before any message content is sent, which is how most real mail servers reject unknown or blocked recipients. The delivered address accepts at both RCPT TO and DATA. This lets you test more than whether your application can submit a message: you can see how it records a rejection, responds to a temporary failure, and surfaces the result to your team.
Test your actual email flow
- Choose a scenario and copy its address from Mailbox Simulator.
- Use that address as the recipient in the application flow you want to test, such as a receipt or account notification.
- Check your sender’s logs or provider dashboard, then verify how your application handles the reported outcome.
You can add a label to an address to identify a test run in your own logs. For example, hardbounce+order123@mailboxsimulator.com produces the same hard-bounce response as the unlabelled address.
Know what the simulator does
The delivered address accepts and discards the message; it does not provide an inbox. Mailbox Simulator does not send bounce emails or event reports. For rejected and deferred messages, inspect the results reported by your sending provider.
Temporary failures return the same response on every retry. Your provider decides when to retry and how to report the final result. That predictability makes it easier to repeat a test after changing your application’s handling.