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Email Bounce Analyzer

Understand Why an Email Bounced

Paste an email bounce message or SMTP error to find out what went wrong and how to fix it. Everything is analyzed locally in your browser.

Bounce Message
Paste a raw bounce, an SMTP response, or a full Delivery Status Notification (DSN).
0 characters Processed locally — never stored or uploaded.

Paste a bounce message above and click Analyze Bounce to see the structured report.

Try one of the example bounces to see how the tool works.

Privacy first
Bounce messages can contain email addresses and other sensitive details. This tool processes everything locally in your browser — your pasted text is never sent to a server, never stored, and never executed.

What Is an Email Bounce?

A bounce is a Delivery Status Notification returned when an email cannot be delivered. The receiving mail server — or a relay along the way — sends back a message explaining why. Bounce messages are packed with useful clues: SMTP reply codes, enhanced status codes, DSN fields and diagnostic text. An email bounce analyzer decodes those clues so you can fix the underlying problem instead of guessing.

Hard Bounce vs Soft Bounce

  • Hard bounce — a permanent failure. The address does not exist, the domain has no mail server, or the message is rejected outright. Retrying the same address will likely fail again; clean it from your list.
  • Soft bounce — a temporary failure. A full mailbox, a busy server or a greylisting response. Retrying later often works, but persistent soft bounces still deserve attention.

SMTP Status Codes & Enhanced Codes

Every SMTP reply starts with a three-digit status code: 421, 450, 550, and so on. The first digit tells you the outcome — 2 = success, 4 = temporary failure, 5 = permanent failure.

  • 4xx (temporary). Try again later — the server was busy, the mailbox was locked, or delivery was deferred.
  • 5xx (permanent). The message failed for good — unknown user, mailbox full, message too large, or a policy rejection.
  • Enhanced codes (e.g. 5.1.1). A more precise dotted code that pinpoints the exact class of failure, like a bad destination mailbox.

Common Bounce Reasons

Recipient problems
User unknown, mailbox does not exist, invalid recipient, address rejected, domain does not exist.
Mailbox problems
Mailbox full, quota exceeded, storage limit reached — usually temporary until the recipient clears space.
Server & network problems
Connection timeout, connection refused, DNS failure, temporary server failure, remote server unavailable.
Message problems
Message too large, attachment too large, invalid or malformed message content.
Policy & security problems
Message rejected, sender/IP/domain blocked, SPF, DKIM or DMARC failure, rate limiting.
Temporary delivery problems
Greylisting, temporary deferral, recipient server busy, transient DNS or network issues.

How to Reduce Email Bounces

  1. Verify addresses before sending. Use an email verifier to remove invalid addresses in advance.
  2. Remove hard-bounced addresses promptly. Repeated sends to invalid addresses hurt your sender reputation.
  3. Authenticate your domain. Publish valid SPF, DKIM and DMARC records so you are not rejected for security reasons.
  4. Respect rate limits. Space out deliveries and honor per-provider sending limits.
  5. Keep messages reasonable. Compress images and avoid attachments that exceed recipient size limits.

Monitor & Improve Delivery with MailCannon

MailCannon helps you track what happens after you send: analyze bounces like the ones above, check your email for spam-risk signals before sending, and preview your HTML on desktop and mobile.

Preview before you send
HTML Email Previewer
Reduce spam risk
Email Spam Checker
Verify an address
Email Checker
Keep templates lean
Email Size Checker

Frequently asked questions

Craft the perfect subject
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Generate a fresh template
AI HTML Email Generator
Improve your preheader
Preheader Generator

About This Analyzer

This analyzer uses deterministic, rule-based parsing. It detects SMTP reply codes and enhanced status codes, reads DSN fields, and matches known bounce patterns — always from the text you provide. It never fabricates measurements or claims a code always has one universal meaning across every provider, and it does not guarantee whether an email will be delivered. When the message is ambiguous or incomplete, it says so.