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ProtocolCodes
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

Related Status Codes

SMTP 421 SMTP 450 SMTP 451 SMTP 550 SMTP 554

Related Terms

TLS Handshake SMTP Relay SPF, DKIM, and DMARC