SMTP
Why Is My Email Bouncing?
Email bounce codes follow a structured pattern: 4xx for temporary failures (soft bounces) and 5xx for permanent failures (hard bounces). This decision tree helps you diagnose the root cause of delivery failures and decide whether to retry or give up.
Decision Steps
Is the failure code in the 5xx range (permanent failure)?
Does the error indicate the recipient mailbox or address does not exist?
Does the error mention the address format or mailbox name is invalid (e.g., illegal characters, bad syntax)?
Is the receiving server indicating it is temporarily unavailable or overloaded?
Is the mailbox full or over quota?
Possible Outcomes
421-service-unavailable
Server temporarily unavailable — retry later
450-mailbox-unavailable
Mailbox full or locked — soft bounce
451-local-error
Server processing error — retry after delay
550-mailbox-not-found
Address does not exist — hard bounce
553-mailbox-name-not-allowed
Invalid address format — hard bounce
554-transaction-failed
Policy rejection — spam or auth failure