1
Paste the bounce code or full bounce message
Enter a short code like 550 5.1.1 or paste the full delivery status notification with Status and Diagnostic-Code fields.
Email Deliverability
Free Email Bounce Code Lookup
An email bounce code is the SMTP and DSN status a receiving mail server returns when it defers or rejects a message. This free lookup tool translates codes like 550 5.1.1 or 421 4.7.0 into plain English so you can see why delivery failed and what to do next.
Paste a short SMTP status or a full delivery status notification and the parser will extract the code, classify it as a soft or hard bounce, and suggest remediation steps. If you need to inspect authentication on a real message afterward, jump to the Email Header Analyzer. For production workflows, MailParse turns inbound email events into structured JSON for your app.
Common email bounce codes reference
A quick table for the most common soft-bounce and hard-bounce patterns.
| Code | Class | Meaning |
|---|---|---|
| 421 4.7.0 | Temporary | Rate limit, greylisting, or temporary policy deferral |
| 450 4.2.0 | Temporary | Mailbox unavailable for now or soft bounce |
| 451 4.3.0 | Temporary | Receiving server had a local processing error |
| 452 4.2.2 | Temporary | Mailbox or system storage temporarily full |
| 550 5.1.1 | Permanent | Mailbox does not exist or recipient is invalid |
| 552 5.2.2 | Permanent | Mailbox full or quota exceeded |
| 553 5.1.3 | Permanent | Address syntax is invalid or malformed |
| 554 5.7.1 | Permanent | Security, spam, relay, or policy rejection |
How to work a bounce problem
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Review the extracted SMTP and DSN status
The tool extracts the numeric status, classifies it as temporary or permanent, and identifies whether the issue is more likely sender-side, recipient-side, or receiver infrastructure.
3
Use the remediation checklist
Follow the recommended next actions to decide whether to retry, suppress the recipient, fix authentication, reduce message size, or contact the receiving provider.
Frequently asked questions
What does an email bounce code mean?
An email bounce code is the SMTP and DSN status returned by a receiving mail server when it cannot accept a message. Codes beginning with 4 mean a temporary problem that may succeed later, while codes beginning with 5 mean a permanent failure that usually needs a fix before retrying.
What is the difference between 4.x and 5.x SMTP errors?
A 4.x.x enhanced status or 4xx SMTP code is a soft bounce, which means delivery was deferred and retrying may work. A 5.x.x enhanced status or 5xx SMTP code is a hard bounce, which means the address, policy, authentication, or message state must usually change before another attempt.
Why do providers show both a code like 550 and a code like 5.1.1?
The three-digit SMTP code is the classic protocol response, while the enhanced status code adds more detail about the reason. For example, 550 says the failure is permanent, and 5.1.1 adds that the problem is an invalid recipient mailbox.
Should I retry a 421 4.7.0 bounce?
Usually yes, but with normal retry backoff rather than immediate manual resends. 421 4.7.0 often means throttling, greylisting, or a temporary policy deferral, so repeated fast retries can make the problem worse.
How do I fix 550 5.7.1 or 5.7.26 errors?
Start by checking SPF, DKIM, and DMARC alignment, then review sender reputation, PTR records, TLS support, and message content. These codes usually mean the receiver blocked the mail for authentication, policy, or spam-related reasons rather than because the mailbox is invalid.
Related tools
Email Header Analyzer
Inspect real message headers, authentication verdicts, and hop-by-hop delivery details.
SPF Record Checker
Validate your SPF record for syntax issues, lookup limits, and sender authorization gaps.
DKIM Checker
Look up selector records and verify that DKIM signing keys are published correctly.