What is a Non-Delivery Report (NDR)?

You're staring at a bounce message in your mailbox thinking: what does "550 5.1.1 user unknown" actually mean? That's a Non-Delivery Report (NDR), also called a Delivery Status Notification (DSN). It's the email system's way of telling you something went wrong.

Here's what's inside: A human-readable section explaining why delivery failed. Machine-readable codes that tell systems what type of problem it was. Details about where the message got stuck and when. Sometimes even the original message itself so you know which email bounced.

The main thing to understand is that NDRs follow a standard format defined in RFC 3464. This means you're seeing a consistent structure whether you're using Gmail, Outlook, or any other mail server. They all use the same basic parts.

Here's the critical detail: NDRs get sent to your Return-Path address, not your From address. If you're not monitoring the right inbox, you'll miss these reports entirely. That's why setting up your Return-Path correctly matters.

SMTP codes (those five-digit numbers) are the real language here. A 5xx code means permanent failure. A 4xx code means temporary. The second digit tells you if it's about syntax, information, connection, authentication, or something else. When you see 550 5.1.1, that's specifically "permanent failure, invalid recipient".

So what do you do when you get an NDR? First, don't suppress the address immediately. Check if it's a hard bounce or soft bounce. A 5xx means hard (permanent). A 4xx might be temporary like a full mailbox. You handle these very differently in your list management.

The next move: Read your NDRs regularly and use them to clean your list intelligently. You can check raw bounce codes with our free email header analyzer if you need to decode a tricky one.