Non-delivery report (NDR)

Why NDRs happen

Your email hits a dead end. Maybe the recipient's address doesn't exist anymore. Maybe their inbox is full. Maybe their server thinks you're spam. The receiving server can't or won't deliver it, so it sends you an NDR explaining why.

These reports aren't just annoying notifications. They're diagnostic tools. The NDR includes an error code and explanation that tells you exactly what went wrong. "User unknown" means bad address. "Mailbox full" means they've hit their storage limit. "Rejected by spam filter" means deliverability problems on your end.

If you're getting a lot of NDRs, that's a red flag. High bounce rates damage your sender reputation. Internet service providers notice when your emails keep bouncing, and they start treating all your mail as suspect. A few bounces here and there? Normal. Consistent bounces? You've got a problem.

Types of bounce messages

NDRs come in two flavors, and the difference matters. Hard bounces mean permanent failure. The email address doesn't exist, the domain is invalid, or the server flat-out rejected you. There's no point trying again. That address is dead. Remove it from your list immediately. Continuing to email hard bounces is how you tank your sender reputation.

Soft bounces are temporary problems. Their inbox is full, their server is down, or the message was too large. These might resolve themselves if you try again later. Most email service providers will automatically retry soft bounces a few times before giving up.

Some NDRs also include detailed diagnostic codes. SMTP error codes like "550 5.1.1" (user doesn't exist) or "552 5.2.2" (mailbox full) give you the technical details. You don't need to memorize these, but they're useful when troubleshooting persistent delivery issues.

Common NDR error codes

Here's that information in a table format:

You'll see these show up in the NDR body. 550 errors usually mean permanent failure. The recipient address doesn't exist, or their server is blocking you. Hard bounce territory.

450 or 451 errors indicate temporary problems. The receiving server is overwhelmed, under maintenance, or implementing greylisting. Soft bounce. Try again later.

552 or 553 errors point to mailbox issues. Full inbox, message too large, or attachment restrictions. Sometimes soft, sometimes hard depending on the specific code.

554 errors mean rejected outright. Could be spam filtering, blacklisting, or policy violations. Check your sender reputation and authentication setup.

What to do when you get an NDR

First, read it. The error message usually tells you exactly what's wrong. Don't just delete it and move on.

For hard bounces, remove that email address from your list. Don't keep trying. If you're seeing lots of hard bounces, your list is stale. Time to clean it up or verify addresses before sending.

For soft bounces, most email systems retry automatically. If it's truly important, wait a few hours and try again manually. But if the same address soft bounces repeatedly (say, five times), treat it like a hard bounce and remove it.

For spam-related rejections, check your email authentication setup. Make sure you've got proper SPF, DKIM, and DMARC records. Review your content for spam triggers. If you're on a blacklist, find out which one and follow their delisting process.

For mailbox full errors? Not much you can do except try again later. Some people just don't manage their inboxes. If it's someone you know personally, maybe mention it to them through another channel.

Frequently Asked Questions About Handling NDRs (Non-Delivery Reports)

What bounce rate should I be concerned about? 

Anything over 2% deserves attention. If your bounce rate exceeds 5%, you have a serious deliverability problem that needs immediate action. Most email platforms track bounce rates automatically in their analytics dashboard.

How often should I clean my email list? 

Remove hard bounces immediately after they occur. For soft bounces, flag addresses that repeatedly fail and remove them after 3-5 attempts. If you're building a list from external sources, use email verification services before importing (though purchasing email lists is strongly discouraged).

What authentication do I need for email delivery? 

SPF, DKIM, and DMARC are essential, not optional. These protocols prove you're a legitimate sender and prevent spammers from impersonating your domain. Without proper authentication, your deliverability will suffer significantly.

What should I do if multiple emails bounce from the same domain? 

This pattern suggests the domain may be blocking you specifically. If reaching users at that domain is important for your business, contact their postmaster team to investigate and resolve the issue.

Should I use double opt-in for new subscribers? 

Yes. Double opt-in requires people to confirm their email address before being added to your list. This catches typos and fake addresses before they cause bounce problems and ensures subscribers genuinely want your emails.

Can I increase my email volume quickly? 

No. Sudden spikes in email volume trigger spam filters at most ISPs. If you need to ramp up your sending, increase volume gradually over several weeks so email providers recognize you as a legitimate sender rather than flagging you as a potential spammer.

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Related terms

• Bulk deleting 
• Flagged as spam 
• Email alias
• Email attachment