SMTP bounce codes explained

A bounce carries two codes: a three-digit reply code and, usually, an enhanced status code such as 5.1.1. The first digit says whether to retry. The enhanced code says why. The column that matters most is the last one: some permanent rejections are about you, not the recipient.

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Basic reply codes (RFC 5321)

Code	Type	Meaning	What to do
`421`	soft bounce	Service not available, closing the connection.	Retry with backoff. Suppress only if it keeps failing for days. Source
`450`	soft bounce	Mailbox unavailable right now, for example busy or temporarily blocked.	Retry with backoff. Suppress only if it keeps failing for days. Source
`451`	soft bounce	Local error in processing. The action was aborted.	Retry with backoff. Suppress only if it keeps failing for days. Source
`452`	soft bounce	Insufficient system storage.	Retry with backoff. Suppress only if it keeps failing for days. Source
`550`	hard bounce	Mailbox unavailable: not found, no access, or rejected for policy reasons. The enhanced code says which.	Read the enhanced code. 5.1.x means suppress, 5.7.x means fix the sender. Source
`551`	hard bounce	User not local.	Suppress the address. Retrying hurts your reputation. Source
`552`	hard bounce	Exceeded storage allocation.	Treat as a full mailbox. Suppress if it repeats. Source
`553`	hard bounce	Mailbox name not allowed, for example invalid syntax.	Suppress the address. Retrying hurts your reputation. Source
`554`	hard bounce	Transaction failed. A generic permanent rejection, often for policy.	Read the enhanced code and the text before suppressing. Source

Enhanced status codes (RFC 3463 and later)

Code	Type	Meaning	What to do
`4.2.2`	soft bounce	Mailbox full.	Retry with backoff. Suppress only if it keeps failing for days. Source
`4.4.1`	soft bounce	No answer from the receiving host.	Retry with backoff. Suppress only if it keeps failing for days. Source
`4.4.2`	soft bounce	Bad connection: the connection dropped during the transaction.	Retry with backoff. Suppress only if it keeps failing for days. Source
`4.7.0`	soft bounce	Other or undefined security status. Receivers use it for temporary blocks.	Retry with backoff and read the text: it often names a rate or reputation limit. Source
`5.1.1`	hard bounce	Bad destination mailbox address: the mailbox does not exist.	Suppress the address. Retrying hurts your reputation. Source
`5.1.2`	hard bounce	Bad destination system address: the domain does not exist or accepts no mail.	Suppress the address. Retrying hurts your reputation. Source
`5.1.3`	hard bounce	Bad destination mailbox address syntax.	Suppress the address. Retrying hurts your reputation. Source
`5.2.1`	hard bounce	Mailbox disabled, not accepting messages.	Suppress the address. Retrying hurts your reputation. Source
`5.2.2`	soft bounce	Mailbox full, reported as permanent.	Retry later. Suppress if it repeats over several days. Source
`5.2.3`	sender problem	Message length exceeds an administrative limit.	Send a smaller message, for example a link instead of an attachment. Source
`5.3.4`	sender problem	Message too big for the system.	Send a smaller message. Source
`5.7.1`	sender problem	Delivery not authorized, message refused.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.20`	sender problem	No passing DKIM signature found.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.21`	sender problem	No acceptable DKIM signature found.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.22`	sender problem	No valid author-matched DKIM signature found.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.23`	sender problem	SPF validation failed.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.24`	sender problem	SPF validation error, for example too many DNS lookups.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.25`	sender problem	Reverse DNS validation failed.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.7.26`	sender problem	Multiple authentication checks failed.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`5.1.10`	hard bounce	Recipient address has a null MX: its domain accepts no mail.	Suppress the address. Retrying hurts your reputation. Source
`5.7.27`	sender problem	Sender address has a null MX, so it cannot receive replies or bounces.	Do not suppress the recipient. Fix your sending setup, then resend. Source

Provider-specific codes

Code	Type	Meaning	What to do
`550 5.1.1` (Gmail)	hard bounce	The email account does not exist.	Suppress the address. Retrying hurts your reputation. Source
`452 4.2.2` (Gmail)	soft bounce	The account is over its storage quota.	Retry with backoff. Suppress only if it keeps failing for days. Source
`421 4.7.28` (Gmail)	soft bounce	Gmail detected an unusual rate of email from your IP, netblock or domain and is rate-limiting it.	Slow down and retry. Check spam rates in Postmaster Tools. Source
`550 5.7.1` (Gmail)	sender problem	The message was blocked as likely unsolicited.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`550 5.7.26` (Gmail)	sender problem	The sender is unauthenticated: the message failed SPF and DKIM or failed DMARC.	Do not suppress the recipient. Fix your sending setup, then resend. Source
`TS* errors` (Yahoo)	soft bounce	Message temporarily deferred: complaints from Yahoo users, spam-like content, a poorly rated IP or subnet, or unusual traffic.	Let your server retry, and fix the cause before volume goes back up. Source
`5.7.606–649` (Microsoft)	sender problem	Access denied, banned sending IP.	Check the IP for compromise or bad traffic, then request removal through Microsoft’s delist portal. Source

The difference between the three types, and why a 5.7 rejection should never land an address on your suppression list, is in hard bounce vs soft bounce. If a code points at authentication, run the domain health check.

Questions

Is every 5xx code a hard bounce?

No. A 5xx code means the receiver will not accept the message as sent. When the enhanced code is in the 5.7 class, the rejection is about the sender’s authentication or reputation, so suppressing the recipient fixes nothing. Fix the sending setup and the same address accepts mail.

What is the difference between 550 and 5.1.1?

550 is the basic reply code from RFC 5321. 5.1.1 is the enhanced status code from RFC 3463 that follows it and says why: the mailbox does not exist.