Email bounce codes help explain why an email could not be delivered. Understanding these codes allows you to diagnose delivery issues, protect your sender reputation, and take corrective action before problems escalate.
When an email bounces, the receiving mail server returns a code that indicates the reason for the failure. These codes generally fall into two categories: traditional bounce codes and enhanced bounce codes.
Traditional bounce codes are three-digit numeric codes that provide a high-level explanation of the delivery failure. They are divided into two main ranges.
Codes in the 400 range usually indicate soft bounces. These are temporary issues that may resolve on their own, such as server delays or full inboxes. Retrying delivery later often succeeds.
Common examples include:
421 Service not available or transmission channel closed
450 Mailbox unavailable or temporarily busy
451 Temporary processing error
452 Insufficient system storage
Codes in the 500 range typically indicate hard bounces. These are permanent failures where the email cannot be delivered and should not be retried.
Common examples include:
550 Mailbox does not exist or is unavailable
552 Mailbox storage exceeded
553 Invalid mailbox name
554 Transaction failed for an unspecified reason
Hard bounces should be addressed immediately by removing or correcting the affected email addresses.
Enhanced bounce codes provide more specific details about delivery failures. These codes follow a structured format, such as 5.1.1, and help pinpoint the exact cause of the bounce.
5.0.0 Address does not exist
5.1.1 Invalid destination mailbox
5.1.6 Mailbox has moved
5.2.1 Mailbox disabled
5.2.2 Mailbox full
5.2.3 Message length exceeds limits
5.3.1 Mail system storage full
5.3.4 Message too large
5.4.1 No response from host
5.4.3 Routing server failure
5.4.6 Routing loop detected
5.4.7 Delivery time expired
5.5.1 Invalid command
5.5.3 Too many recipients
5.7.1 Delivery not authorized
5.7.5 Cryptographic failure
5.7.7 Message integrity failure
Enhanced bounce codes are especially useful for identifying authentication issues, security blocks, or configuration errors.
In addition to standard codes, mail servers may include custom text explaining the bounce. These messages often provide extra context specific to the receiving server.
Always review both the bounce code and the accompanying message to fully understand the issue.
High bounce rates can negatively impact your sender reputation and deliverability. By identifying whether bounces are temporary or permanent, you can:
Clean invalid email addresses
Adjust sending volume or timing
Fix authentication or configuration issues
Prevent future deliverability problems
Bounce codes are not just error messages. They are diagnostic signals that help you understand how receiving servers view your emails. Interpreting them correctly allows you to maintain healthy sending practices and protect long-term inbox placement.
If you need help interpreting specific bounce codes or resolving repeated failures, the support team is always available.