How are bounce messages generated?

You hit send, the email disappears into the internet, and a few minutes later something comes back with bad news. That's a bounce message. But where exactly did it come from, and what does it actually mean for your list?

Here's how the process works. Your sending server hands the email off to the receiving server. If the receiving server accepts it (you get a 250 OK response), the message is in their queue. At that point, your server's job is done. But then the receiving server tries to deliver the message to the actual mailbox. If that final step fails, the receiving server generates a Delivery Status Notification (DSN) and sends it back to the Return-Path address on the original message. That's the bounce.

The DSN typically includes the original message headers, a plain-language error description, an SMTP status code (like 550 or 421), the timestamp, and which server generated the failure. Your ESP reads all of this automatically and categorizes the bounce for you.

The most important thing the bounce tells you is whether the failure is permanent or temporary. A hard bounce means the address doesn't exist or the domain is gone. It won't work next time either. A soft bounce means something temporary got in the way, like a full mailbox or a slow server. It might work if you try again. Understanding that distinction is what actually tells you what to do next.

Not every bounce comes from the final destination server. Intermediate mail relays can generate one too, if they can't forward the message further. And your own sending server can generate a bounce if it can't connect to the receiving server at all, before anything is accepted.

For your list hygiene, the rule is simple. Hard bounces should be removed immediately. Soft bounces that repeat across multiple send attempts are a signal to suppress the address too. Ignoring bounces drives up your error rate and tells mailbox providers you're not paying attention to your list, which hurts your sender reputation over time.

If you're seeing a spike in bounces and you're not sure whether it's your list, your domain, or something else, our SOS hotline is free and we'll help you figure it out.