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.
Related tools
- Bounce Code Lookup — Look up specific SMTP error codes
- Blacklist Checker — Check if your IP or domain is blacklisted
- SPF Checker — Verify your SPF record
- DKIM Checker — Check your DKIM setup
- DMARC Checker — Validate your DMARC policy
- Deliverability Score — Get an overall deliverability assessment
- Bounce Rate Calculator — Calculate your bounce rate
Frequently Asked Questions
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