Skip to content

SMTP responses: what each one means and what to do

Paste a bounce out of your mail log, or type any part of a code. Every reply code in RFC 5321, every enhanced status code IANA has registered, and Microsoft's own, which are mostly outside both.

Response, code, or fragment of one from RFC 5321, the IANA registry and Microsoft’s own

Try:

Written up in full

The ones that come back most often, and the ones whose wording hides what actually happened. The same 550 means a dead mailbox at one provider and a reputation block at another, and the text beside it is where that lives rather than in the number. Everything else resolves in the lookup above.

4.2.2RFC 3463The mailbox is over quota. Temporary, and the address is still good. Retry with backoff.retry 4.4.1RFC 3463A transport failure, not a mail failure. Retry, but investigate if it sticks to one route.retry 5.1.1RFC 3463The mailbox does not exist. Permanent. Suppress it, because retrying costs reputation.suppress 5.2.1RFC 3463The mailbox exists but is disabled. Suppress it, these turn into spam traps.suppress 5.7.1RFC 3463A deliberate policy rejection by the receiver, usually not about your reputation.review Spamhaus listingBlocklistThe receiver queried Spamhaus and found your IP listed. Delist before sending again.pause TLS handshakeTransportThe connection opened but encryption could not be negotiated. Usually cipher or certificate.retry 4.7.28GmailA throttle, not a block. Slow down for Gmail specifically and the mail still goes.throttle 5.7.1GmailA hard reputation block. Stop sending from this IP or domain and remediate.pause 5.7.26GmailAuthentication failed. SPF or DKIM has to pass and align, and retrying will not help.fix config 4.7.500Microsoft / OutlookThrottling. Deferred rather than refused, so back off per-provider and retry.throttle 5.7.606Microsoft / OutlookYour sending IP is on Outlook's block list. Delist, then fix what caused it.pause DY-001Microsoft / OutlookThe connecting address looks like consumer or dynamic space.fix config RP-001Microsoft / OutlookOutlook is rate limiting your address because of its reputation, not because of a fixed cap.throttle RP-002Microsoft / OutlookThe same rate limit as RP-001, applied to one connection rather than to the address.throttle RP-003Microsoft / OutlookToo many connections open to Outlook.com at once.throttle S3140Microsoft / OutlookMicrosoft's internal reason code on a block. It points at IP reputation.pause SC-001Microsoft / OutlookOutlook refused the message on policy: content or the reputation of the address behind it.review SC-004Microsoft / OutlookBlocked because Outlook recipients complained about mail from this address.pause TS03YahooA 4xx that behaves like a block. Retrying on schedule will not clear it.pause

Where these come from

The reply codes are parsed from RFC 5321 section 4.2.3, the enhanced status codes from the IANA registry, and the Microsoft codes from Microsoft's own NDR reference. Every entry names its source and links back to it.

What none of those registries carry is what to do about a code, which is the only reason anybody looks one up. So every entry also carries an action. Where I have worked the failure myself, the action is what actually cleared it. Where it follows from the code's class and subject, the entry is marked as derived, so you can tell a safe default from a tested one.

Written and maintained by Rastu Singh, an infrastructure engineer working on high-volume email in Tallinn.