Outlook and Microsoft 365 bounce playbook.

Every Exchange Online bounce we cover, from IP bans and throttles to tenant blocks and region errors, with whether it retries, who needs to act, and the fix.

Browse all raw Outlook replies

Every Microsoft code, with the fix.

CodeRetrying or final · Who acts

421 4.7.0Temporary system problemRetryingReceiving server
451 4.4.4 (ATTR5)Temporary server errorRetryingReceiving server
451 4.7.500 (S77719)Server busy (S77719)RetryingReceiving server
452 4.5.3 (ATTR38)Mixed regionsRetryingSender
451 4.4.62 (ATTR35)Wrong regionRetryingReceiving server
451 4.7.650Sender blocked by EOPRetryingSender
550 5.4.1Recipient address rejectedFinalReceiving server
554 5.4.14 (ATTR34)Hop count exceededFinalReceiving server
550 5.7.511Banned senderFinalSender
550 5.7.606Banned sending IPFinalSender
550 5.7.708Traffic not accepted from IPFinalSender
550 5.7.350Tenant policy blockFinalRecipient
550 5.7.64TenantLevelReputationFinalRecipient

13 codes, 7 finalSelect a code for the full fix

More references.

SMTP field manual.

Every SMTP reply code, with real bounces from mailbox providers and spam filters and who needs to act.

Email marketing glossary.

Plain definitions for deliverability, authentication, automation, and list terms.

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