Email Bounce Code Lookup - SMTP Error Code Explained
Enter an SMTP status code or paste a bounce message to get a plain English explanation, common causes, and what to do to fix it.
554 is a generic permanent failure code. The receiving server rejected the transaction but the specific reason depends on the sub-code or message text. Common sub-codes: 5.7.1 (policy), 5.7.26 (authentication).
Common causes:
- Authentication failure (look for 5.7.26, 5.7.1 sub-codes)
- Content blocked by spam filter
- Sending server or domain on a blocklist
- Missing or invalid headers
What to do:
- Read the full bounce message for a sub-code after 554
- For 5.7.26: run an InboxGreen domain check to fix authentication
- For 5.7.1: check blacklists and SPF/DMARC configuration
Common SMTP bounce codes explained
5xx - Permanent Failures
The email was rejected permanently. The problem must be fixed before retrying - resending without changes will result in another bounce.
| Code | Meaning | Quick fix |
|---|---|---|
550 |
Mailbox unavailable or rejected. The most common bounce code - covers blocked senders, non-existent addresses, and policy rejections. | Check if address exists; verify SPF/DKIM/DMARC; check blacklists |
551 |
User not local - the server does not handle mail for this address and is not relaying it. | Verify the email address is correct |
552 |
Storage exceeded - the recipient's mailbox is full. | Wait and retry later, or contact recipient through another channel |
553 |
Mailbox name invalid - the address format is not acceptable to the receiving server. | Double-check the recipient address for typos |
554 |
Transaction failed - a generic permanent rejection. Often includes a sub-code like 5.7.1 (policy) or 5.7.26 (authentication). | Read the full error message for the specific reason; fix SPF/DMARC if 5.7.26 |
550 5.7.26 |
Gmail/Google rejection for missing or failing email authentication. SPF, DKIM, or DMARC is not set up correctly. | Run a free InboxGreen check to find and fix the authentication problem |
550 5.1.1 |
Recipient address does not exist at the destination server. | Remove the address from your list; verify it was entered correctly |
550 5.7.1 |
Policy rejection - the receiving server rejected your message due to a policy rule (blacklist, SPF failure, reputation). | Check blacklists; verify SPF includes your sending server; warm up IP if new |
4xx - Temporary Failures
The email was deferred - the server will retry automatically. If it keeps failing, investigate the cause.
| Code | Meaning | Quick fix |
|---|---|---|
421 |
Service temporarily unavailable - the receiving server is busy or rate-limiting your connection. | Reduce sending rate; your sending server will retry automatically |
450 |
Mailbox temporarily unavailable - could be a greylisting check or a temporary server issue. | Wait for your server to retry; usually resolves within an hour |
451 |
Local processing error - the receiving server encountered a problem on its side. | Your server will retry; if persistent, check SPF and blacklist status |
452 |
Insufficient storage on the receiving server. | Wait and retry later; usually temporary |
What do the sub-codes mean? (e.g. 5.7.26)
Many SMTP responses include a three-part enhanced status code (X.Y.Z) after the main 3-digit code. The first digit matches the main code (4 or 5). The second digit describes the category:
- X.0.Z - Other or undefined status
- X.1.Z - Addressing problem (bad address, unknown recipient)
- X.2.Z - Mailbox problem (full, unavailable)
- X.3.Z - Mail system problem
- X.4.Z - Network and routing problem
- X.5.Z - Mail delivery protocol problem
- X.6.Z - Message content or media problem
- X.7.Z - Security or policy problem (most authentication failures here)
For 5.7.x codes, the issue is almost always related to authentication (SPF, DKIM, DMARC) or reputation. Run a free InboxGreen domain check to identify exactly what is misconfigured.
Authentication-related bounce? Check your domain: