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SMTP response code lookup.
Descifra cualquier respuesta SMTP: los códigos 4xx y 5xx que devuelven los servidores, además de los códigos de estado ampliados como 5.1.1 y 5.7.26. Busca un código o una palabra clave y descubre qué significa y qué debería hacer quien envía. Funciona aquí mismo, en tu navegador.
Filter by reply code (e.g. 421), enhanced status code (e.g. 5.1.1), or a plain-language keyword.
| Code | Class | Meaning | What to do |
|---|---|---|---|
| 211 | Info | System status, or a system help reply. | Informational only — returned in response to a status query, not a delivery outcome. No action needed. |
| 214 | Info | Help message describing how to use the server or its commands. | Informational reply to a HELP command. Nothing to fix on the sending side. |
| 220 | Info | Service ready — the greeting a server sends when it accepts your connection. | This is the normal opening banner. If you never see it, the host isn't accepting SMTP connections on that port. |
| 221 | Info | Service closing the transmission channel after a QUIT. | Normal, graceful end of the session. No action needed. |
| 250 | Success | Requested action completed — the command (MAIL, RCPT, or the message body) was accepted. | The address was accepted at SMTP time. On a catch-all domain a 250 to RCPT does not prove the mailbox exists, so treat it as accepted-but-unconfirmed. |
| 251 | Success | User is not local; the message will be forwarded to another address. | Delivery will be relayed onward. Fine to treat as deliverable; the forwarding target is out of your control. |
| 252 | Success | The server cannot verify the recipient but will accept the message and attempt delivery. | Common when VRFY is disabled or the domain is catch-all. You can't confirm the mailbox from the probe alone — a full verification pipeline scores it instead of guessing. |
| 354 | Info | Start mail input — send the message body, then end with a lone dot on its own line. | Intermediate prompt after DATA. Expected mid-conversation; no action needed. |
| 421 | Temporary failure | Service not available; the server is closing the connection. | Back off and retry later. Often means the host is overloaded, rate-limiting you, or throttling because of your sending reputation — slow your send rate. |
| 450 | Temporary failure | Mailbox unavailable — busy or temporarily blocked (frequently greylisting). | Retry after a delay; greylisting usually clears on the second attempt. Persistent 450s from one provider point to a reputation or rate problem. |
| 451 | Temporary failure | Action aborted — a local error in the receiving server's processing. | Transient on the receiver's side. Retry later; if it persists for a whole domain it may be a policy deferral, not a real server error. |
| 452 | Temporary failure | Insufficient system storage, or too many recipients in one transaction. | Retry later, and split large RCPT batches into smaller transactions. Not a reason to suppress the address. |
| 500 | Permanent failure | Syntax error — the server did not recognise the command. | A protocol-level bug in the sending client, not a recipient problem. Check the exact bytes your client is writing to the socket. |
| 501 | Permanent failure | Syntax error in the command's parameters or arguments. | Often a malformed MAIL FROM or RCPT TO address. Validate address syntax before you send it to the server. |
| 502 | Permanent failure | Command not implemented by this server. | Common for VRFY and EXPN, which most servers disable. Don't rely on those commands to confirm a mailbox. |
| 503 | Permanent failure | Bad sequence of commands — you sent something out of order. | For example RCPT before MAIL, or a command before EHLO. Fix the ordering in the sending client. |
| 504 | Permanent failure | Command parameter not implemented. | The server understood the command but not an argument you supplied (e.g. an unsupported AUTH mechanism). Adjust the parameter. |
| 521 | Permanent failure | The host does not accept mail at all (RFC 7504). | The domain has advertised that it never receives email. Any address there will hard-bounce — suppress it. |
| 530 | Permanent failure | Authentication required, or you must issue STARTTLS first. | The server refused the command until you authenticate or start TLS. Configure credentials / TLS on the sending side; it isn't a recipient fault. |
| 550 | Permanent failure | Mailbox unavailable — most often the address does not exist, or the message was rejected by policy. | The classic hard bounce. If it's 'user unknown' / 'no such mailbox', suppress the address permanently. If it cites a policy or blocklist, fix your reputation or authentication instead. |
| 551 | Permanent failure | User not local; the server names where to send instead but won't relay for you. | Send to the address the server points to. Rare in modern hosted email. |
| 552 | Permanent failure | Mail action aborted — the message exceeded the mailbox's storage allocation, or the message was too large. | For an over-quota mailbox this is effectively undeliverable now; the temporary form is 452. Reduce message size if that's the cause. |
| 553 | Permanent failure | Mailbox name not allowed — the address is syntactically unacceptable to the server. | Usually a malformed or disallowed local-part. Validate and correct the address; don't keep retrying it. |
| 554 | Permanent failure | Transaction failed — a catch-all rejection, commonly a blocklist or content/policy refusal. | Read the text after the code: it names the real reason (spam block, blocklisted IP, no SMTP service). Fix sending reputation or authentication rather than retrying blindly. |
| 4.2.2 | Temporary failure | Mailbox full — the recipient is over their per-mailbox quota (persistent transient). | Retry for a while; it may clear if the user frees space. If it never clears, treat the address as a soft-bounce that has aged into unreachable. |
| 4.3.1 | Temporary failure | Mail system full — the receiving system is out of storage. | A receiver-side capacity problem, not your address list. Retry later. |
| 4.4.1 | Temporary failure | No answer from host — the connection attempt to the destination wasn't answered. | The target server was unreachable or too busy to respond. Retry later; if a whole domain stays dark, its mail service may be down. |
| 4.7.1 | Temporary failure | Delivery not authorised, message refused — a temporary policy block. | Frequently greylisting or a reputation-based deferral. Retry on a normal schedule; repeated 4.7.1 across providers means you should improve IP/domain reputation and authentication. |
| 5.1.1 | Permanent failure | Bad destination mailbox address — the recipient does not exist. | A definitive hard bounce. Suppress the address immediately; leaving it on the list drives up your bounce rate and hurts reputation. |
| 5.1.2 | Permanent failure | Bad destination system address — the recipient's domain can't be found or can't receive mail. | The domain, not just the mailbox, is invalid. Suppress the address and check for a typo in the domain part. |
| 5.2.1 | Permanent failure | Mailbox disabled — it exists but is not accepting messages. | Often a suspended or deactivated account. Treat as undeliverable and suppress; it won't recover on retry. |
| 5.2.2 | Permanent failure | Mailbox full, reported as permanent by this server. | Some servers return the quota problem as a hard failure. Stop sending to it; re-verify later before re-adding. |
| 5.4.1 | Permanent failure | No answer from host — or, at Microsoft 365, 'Recipient address rejected: Access denied'. | In practice this is Microsoft's usual reply for an address that doesn't exist or is blocked. Treat as a hard bounce and suppress. |
| 5.7.1 | Permanent failure | Delivery not authorised, message refused — a permanent policy rejection. | You were blocked: blocklisted IP, relay denied, or filtered as spam. Fix authentication and reputation; the recipient address itself may be fine. |
| 5.7.26 | Permanent failure | Multiple authentication checks failed — the message failed more than one of SPF, DKIM, and DMARC. | Gmail and others reject unauthenticated bulk mail with this. Publish and align SPF, DKIM, and DMARC for your sending domain before resending. |
The first digit is the outcome: 2xx succeeded, 3xx is an intermediate prompt, 4xx is a temporary failure (retry), and 5xx is permanent (suppress). Enhanced codes like 5.1.1 add a precise reason on top.
Cómo leer una respuesta SMTP
Every SMTP response starts with a three-digit reply code whose first digit is the whole story for deliverability. The full códigos de respuesta SMTP reference walks through each one, but three bands cover almost every case you'll meet:
- 2xx — aceptado
- El comando tuvo éxito. Un 250 significa que se aceptó al destinatario, aunque en un dominio catch-all eso no prueba que el buzón exista.
- 4xx — reintentar más tarde
- Un fallo temporal, como el greylisting o un buzón lleno. Reintenta con una cadencia normal; no suprimas la dirección solo por un 4xx.
- 5xx — permanente
- Un fallo duro: el buzón no existe o el mensaje fue rechazado por política. Un 550 o un 5.1.1 debería suprimirse para que deje de inflar tu tasa de rebotes.
Del código de respuesta a la categoría de rebote
The reply code is what turns a failed send into a bounce, and how you classify it decides what happens next. Mapping each code to hard, soft, or block is the heart of clasificación de rebotes de correo — a permanent 5xx means suppress, a transient 4xx means retry, and a policy rejection means fix your reputation rather than the address. The cleanest way to avoid the hard bounces entirely is to remove dead addresses before you send, so the server never has to answer 550.
Búsqueda de códigos de respuesta SMTP — Preguntas frecuentes
Detén los 550 antes de que ocurran
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