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.
550 5.1.1 means the email address you are trying to send to does not exist at the destination server. The mailbox has been deleted, was never created, or you have a typo in the address.
Common causes:
- Email address was deleted or deactivated
- Typo in the recipient email address
- The domain exists but the specific mailbox does not
What to do:
- Remove this address from your list
- Double-check for typos in the address
- Contact the person through another channel to get their current email
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: