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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.

Mailer Daemon Decoder

Paste a bounce message or error code to understand what went wrong and how to fix it

Or try an example:

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What Is a Mailer Daemon?

A "mailer daemon" isn't a virus or a hacker — it's simply the automated system on mail servers that handles delivery failures. When an email can't be delivered, the mailer daemon sends you a bounce message explaining what went wrong. Think of it as the postal service returning a letter marked "address unknown."

The word "daemon" comes from Unix computing — it refers to a background process that runs automatically without human intervention. Every mail server has one, and they all do the same thing: try to deliver your email, and if they can't, tell you why.

Understanding Bounce Types

Hard Bounce

Permanent failure. The email will never be delivered to this address. Remove it from your list. Common causes: address doesn't exist, domain not found.

Soft Bounce

Temporary failure. The email might be delivered on retry. Your ESP handles this automatically. Common causes: mailbox full, server busy, rate limiting.

Policy Block

Rejected by the server's rules. Fix your authentication, reputation, or configuration. Common causes: failed SPF/DKIM, blacklisted IP.

Content Block

The email content triggered a spam filter. Review your content, links, and attachments. Use our spam word checker to identify trigger words.

SMTP Error Code Quick Reference

2xxSuccess — email accepted for delivery
4xxTemporary failure — try again later (soft bounce)
5xxPermanent failure — do not retry (hard bounce)

Enhanced status codes (like 5.1.1) provide more detail:

  • x.1.x— Address-related (mailbox doesn't exist, domain not found)
  • x.2.x — Mailbox-related (full, disabled, not accepting mail)
  • x.3.x — Mail system issue (server problem)
  • x.4.x— Network/routing (can't reach the server)
  • x.5.x — Protocol issue (mail system misconfiguration)
  • x.7.x — Security/policy (authentication, permissions)

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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