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Mailer Daemon Decoder

Paste any bounce message, mailer-daemon reply, or SMTP error code to get a plain-English explanation of what went wrong, whether it's a hard bounce, soft bounce, or policy block, and exactly what steps to take to fix it. Covers 550 errors, 421 rate limits, spam blocks, authentication failures, blacklisting, and more.

About this tool

Why you're getting bounce messages

Every time you send an email that can't be delivered, the receiving mail server sends back a "bounce message" — also called a mailer-daemon reply, NDR (Non-Delivery Report), or DSN (Delivery Status Notification). These messages contain error codes and technical jargon that explain why your email was rejected.

The problem? Most people see "550 5.1.1" and have no idea what it means. This tool decodes those cryptic messages into plain English and tells you exactly what to do about it.

How this decoder works

Paste the full bounce message or just the error code into the text box. The decoder matches patterns in the text against known SMTP error codes and bounce reasons. It identifies whether you're dealing with a hard bounce (permanent), soft bounce (temporary), policy block, content block, or technical issue.

For each match, you'll see a clear explanation of what happened and step-by-step instructions for resolving it.

Common bounce messages explained

Here are the most frequent bounce messages email marketers encounter:

  • 550 User not found — The email address doesn't exist. Remove it from your list.
  • 452 Mailbox full — Recipient's inbox is full. Your ESP will retry automatically.
  • 550 Blocked as spam — Content or reputation issue. Review your spam triggers and deliverability setup.
  • 421 Too many connections — You're sending too fast. Slow down or let your ESP handle throttling.
  • 550 SPF/DKIM failure — Authentication issue. Check your SPF, DKIM, and DMARC records.

Hard bounces vs. soft bounces

Understanding the difference is critical for list management:

  • Hard bounces are permanent failures — the address doesn't exist or has been permanently blocked. Remove these immediately. Continuing to send to hard bounces damages your sender reputation.
  • Soft bounces are temporary — the mailbox is full, the server is busy, or you're being rate limited. Your ESP automatically retries soft bounces for 24-72 hours. If they keep failing, they'll eventually be treated as hard bounces.

Most ESPs (including Sequenzy) automatically handle bounce classification and list cleanup. But understanding the reason behind each bounce helps you prevent future deliverability problems.

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Frequently Asked Questions

What is a mailer daemon?

A mailer daemon is an automated system on mail servers that handles email delivery and bounces. When your email can't be delivered, the mailer daemon sends you a bounce message explaining why. It's not a virus or a hacker — 'daemon' is a Unix computing term for a background process that runs automatically. Think of it as the postal service returning an undeliverable letter with a reason stamped on it.

Why did I get a mailer daemon message?

You received a mailer daemon message because an email you sent couldn't be delivered. Common reasons include: the email address doesn't exist (typo or deleted account), the recipient's mailbox is full, your email was flagged as spam, your sending server failed authentication checks, or the recipient's server is temporarily unavailable. Paste the full bounce message into this decoder to find out the specific reason and what to do about it.

What does error code 550 mean?

A 550 error means the receiving server permanently rejected your email. It's the most common hard bounce code. The most frequent cause is 'user not found' — the email address doesn't exist. But 550 can also mean your email was blocked by a spam filter, your domain failed authentication, or the recipient's server has a policy blocking your messages. The text after the 550 code provides specific details about why it was rejected.

Should I remove bounced addresses from my list?

Yes for hard bounces (5xx errors) — remove them immediately. Continuing to send to non-existent addresses damages your sender reputation and can get you blacklisted. For soft bounces (4xx errors), let your ESP handle automatic retries. If an address soft-bounces consistently over multiple campaigns (typically 3-5 times), remove it. Most ESPs automatically suppress addresses after repeated bounces.

What's the difference between a 4xx and 5xx error?

4xx errors are temporary failures (soft bounces) — the email might be delivered if you retry later. The server is saying 'not right now, but try again.' Common examples: 421 (too many connections), 450 (mailbox busy), 452 (mailbox full). 5xx errors are permanent failures (hard bounces) — the email will never be delivered to this address. The server is saying 'no, and don't try again.' Common examples: 550 (user not found), 553 (invalid address), 554 (rejected).

Why are my emails being blocked as spam?

Emails get blocked as spam for several reasons: your sending IP or domain is on a blacklist, you're missing SPF/DKIM/DMARC authentication, your content contains spam trigger words or patterns, you have a high complaint rate, you're sending to invalid addresses (spam traps), or the recipient has personally marked your emails as spam. Start by checking your authentication setup, then review your content with a spam word checker, and make sure you're only sending to opted-in subscribers.

What does 'relay access denied' mean?

Relay access denied means the mail server you're connecting to won't forward (relay) your email to the intended recipient. This usually means one of three things: the server doesn't handle email for that domain, your sending server isn't authorized to use it as a relay, or the domain's DNS records are misconfigured. If you're using an ESP, this is rare and usually indicates a problem with the recipient's mail setup rather than yours.

How do I fix authentication failure bounces?

Authentication failures (SPF, DKIM, or DMARC failures) mean the receiving server couldn't verify that you're authorized to send from your domain. To fix: 1) Check your SPF record includes your ESP's sending servers, 2) Verify DKIM signing is properly configured in your ESP settings, 3) Make sure your DMARC policy isn't set to 'reject' if your authentication isn't fully aligned, 4) If you recently changed ESPs, you need to update all DNS records. Use our SPF Checker, DKIM Checker, and DMARC Checker tools to diagnose specific issues.

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