Email Bounce Code Lookup - SMTP Error Code Explained

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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.

Enter just the code (e.g. 550) or paste the full bounce message. Common codes: 421, 450, 550, 551, 552, 553, 554.

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.

CodeMeaningQuick 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.

CodeMeaningQuick 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:

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