SMTP and email delivery codes
An email bounce reports why it failed as two numbers: a three-digit reply code and a three-part status code, written together as "550 5.7.1". Find yours below, or search for it directly — including in the forms a bounce prints it, such as "5.7.1" or "550 5.1.1".
Security or policy status
- SMTP error 5.7.1Delivery not authorized, message refusedSomebody decided you are not permitted to send to that recipient, and refused the message outright. It arrives…
- SMTP error 5.7.23SPF validation failedThe receiving server looked up the SPF record for your domain, compared it against the IP address the…
- SMTP error 5.7.8Authentication credentials invalidThe username and password presented were rejected, so nothing was ever submitted. It appears as 535 5.7.8 in…
- SMTP error 5.7.26Multiple authentication checks failedMore than one authentication check failed on the same message and the receiver treated the combination as grounds…
- SMTP error 5.7.24SPF validation errorThis is not SPF saying no. It is SPF unable to produce an answer at all: the receiver…
- SMTP error 5.7.20No passing DKIM signature foundNot one DKIM signature on this message verified. Either it carried no DKIM-Signature header at all, or every…
- SMTP error 5.7.25Reverse DNS validation failedThe receiving server took the IP address your mail arrived from, looked up its PTR record and was…
- SMTP error 5.7.13User Account DisabledThe password was right and the account is still switched off. A reply of 525 5.7.13 is the…
- SMTP error 5.7.0Other or undefined security statusThis is the catch-all of the security and policy family. Something in the security layer refused the transaction…
- SMTP error 5.7.22No valid author-matched DKIM signature foundEverything about the signature is sound except whose it is. The message carries a DKIM signature that verifies…
- SMTP error 5.7.9Authentication mechanism is too weakThe credentials may be entirely correct; the method used to present them is not good enough for this…
- SMTP error 5.7.21No acceptable DKIM signature foundThe message carried at least one DKIM signature that verified, and the receiver declined it anyway because none…
- SMTP error 5.7.10Encryption NeededYou attempted to authenticate over an unencrypted connection and the server declined to accept a password in the…
- SMTP error 5.7.27Sender address has null MXYour own sending domain publishes a null MX, the record that formally states the domain accepts no email.…
- SMTP error 5.7.29ARC validation failureARC is the mechanism that lets a forwarder or a mailing list record that a message authenticated correctly…
- SMTP error 5.7.2Mailing list expansion prohibitedYou are allowed to email the organisation but not that particular list. A list manager checked who you…
- SMTP error 5.7.11Encryption required for requested authentication mechanismA narrower and now largely historical cousin of 5.7.10: the specific mechanism chosen may only be used once…
- SMTP error 5.7.7Message integrity failureA server that was supposed to be able to validate this message could not, because what arrived does…
- SMTP error 5.7.4Security features not supportedOne side asked for a security feature the other side cannot provide, and the request was refused with…
- SMTP error 5.7.14Trust relationship requiredThe submission server required an established trust with another system in order to reach the message content, and…
- SMTP error 4.7.12A password transition is neededA temporary refusal telling the client that the account has to be moved onto the mechanism it just…
- SMTP error 5.7.30REQUIRETLS support requiredThe message was submitted carrying a REQUIRETLS flag, an explicit instruction that it must only travel over TLS…
- SMTP error 5.7.16Message is too big for the specified priorityA specialist code from the SMTP priority extension, and one almost no UK business will ever see. The…
Address status
- SMTP error 5.1.1Bad destination mailbox addressThere is no mailbox by that name on the receiving system. The part to the left of the…
- SMTP error 5.1.2Bad destination system addressThe domain after the @ cannot receive mail. Either it does not exist in DNS, or it exists…
- SMTP error 5.1.3Bad destination mailbox address syntaxThe address is not a valid address. Not wrong, not dead: malformed. Something in the string breaks the…
- SMTP error 5.1.8Bad sender's system addressThe problem is your sender domain, not the address you wrote to. The receiving server has looked at…
- SMTP error 2.1.5Destination address validThis is a success, not a fault. It confirms the address you sent to is a real, accepted…
- SMTP error 5.1.10Recipient address has null MXThe domain you are writing to publishes a null MX, a single mail record at priority 0 pointing…
- SMTP error 5.1.6Destination mailbox has moved, No forwarding addressThe address used to work and deliberately no longer does. The receiving system still recognises it, knows the…
Mailbox status
- SMTP error 5.2.2Mailbox fullThe recipient's mailbox has no room left. It arrives as 552 5.2.2 worded as mailbox full or over…
- SMTP error 5.2.3Message length exceeds administrative limitYour message is bigger than this specific mailbox is allowed to receive. It comes back as 552 with…
- SMTP error 5.2.1Mailbox disabled, not accepting messagesThe mailbox is real and is not taking delivery. Somebody has put it into a state where it…
- SMTP error 4.2.4Mailing list expansion problemThe address you wrote to is a distribution list, and the system could not work out who is…
Network and routing status
- SMTP error 4.4.1No answer from hostYour mail server tried to open a connection to the destination and got silence. Not a refusal, an…
- SMTP error 5.4.4Unable to routeThe mail system could not work out where to send the message next. In Microsoft 365 this is…
- SMTP error 4.4.2Bad connectionThe conversation began and then broke. The connection was open, both servers had exchanged commands, and it fell…
- SMTP error 4.4.5Mail system congestionThe far end is busy and is asking you to come back later, reported as 451 4.4.5. Nothing…
- SMTP error 4.4.6Routing loop detectedThe message went round in a circle. Two systems each believed the other was responsible for the address…
- SMTP error 5.4.3Directory server failureA lookup the mail system depends on did not answer, so it could not work out what to…
Mail system status
- SMTP error 5.3.4Message too big for systemThe message is larger than the whole system will carry, not just larger than one mailbox is allowed.…
- SMTP error 4.3.1Mail system fullThe receiving system has run out of storage for mail as a whole, not for one user. It…
- SMTP error 5.3.2System not accepting network messagesThe host answered and then said it is not taking mail. That is the distinguishing feature: something replied.…
- SMTP error 5.3.0Other or undefined mail system statusA catch-all. The destination exists and normally takes mail, but something went wrong on it that the reporting…
Mail delivery protocol status
- SMTP error 5.5.1Invalid commandThe client sent an SMTP command the server would not accept, either out of sequence or one it…
- SMTP error 5.5.2Syntax errorThe server could not parse the line it received. Not the wrong command at the wrong moment, which…
- SMTP error 4.5.3Too many recipientsMore recipients were named than the receiving system would accept in one transaction, reported as 451 4.5.3. It…
Message content or media status
Every other SMTP status code
These are the remaining codes in the registry. Most are the ones RFC 3463 registers in template form, where the leading digit is chosen by the reporting server rather than fixed by the specification, so we have not assigned one — but the registry defines each condition, and its official definition is reproduced here in full.
| Code | Official name | What the specification says | Defined in |
|---|---|---|---|
| SMTP error X.2.0 | Other or undefined mailbox status | The mailbox exists, but something about the destination mailbox has caused the sending of this DSN. | RFC 3463 |
| SMTP error X.1.9 | Message relayed to non-compliant mailer | The mailbox address specified was valid, but the message has been relayed to a system that does not speak this protocol; no further information can be provided. | RFC 3886 |
| SMTP error X.7.6 | Cryptographic algorithm not supported | A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported. | RFC 3463 |
| SMTP error X.1.7 | Bad sender's mailbox address syntax | The sender's address was syntactically invalid. This can apply to any field in the address. | RFC 3463 |
| SMTP error X.3.3 | System not capable of selected features | Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another. | RFC 3463 |
| SMTP error X.3.5 | System incorrectly configured | The system is not configured in a manner that will permit it to accept this message. | RFC 3463 |
| SMTP error X.4.0 | Other or undefined network or routing status | Something went wrong with the networking, but it is not clear what the problem is, or the problem cannot be well expressed with any of the other provided detail codes. | RFC 3463 |
| SMTP error X.4.7 | Delivery time expired | The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live value specified by the sender of the message was exceeded. If possible, the code for the actual problem found when delivery was attempted should be returned rather than this code. | RFC 3463 |
| SMTP error X.5.5 | Wrong protocol version | A protocol version mis-match existed which could not be automatically resolved by the communicating parties. | RFC 3463 |
| SMTP error X.6.0 | Other or undefined media error | Something about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes. | RFC 3463 |
| SMTP error X.6.2 | Conversion required and prohibited | The content of the message must be converted before it can be delivered and such conversion is not permitted. Such prohibitions may be the expression of the sender in the message itself or the policy of the sending host. | RFC 3463 |
| SMTP error X.7.17 | Mailbox owner has changed | This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system is able to determine that the intended recipient mailbox has not been under continuous ownership since the specified date-time. | RFC 7293 |
| SMTP error X.7.18 | Domain owner has changed | This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system wishes to disclose that the owner of the domain name of the recipient has changed since the specified date-time. | RFC 7293 |
| SMTP error X.7.19 | RRVS test cannot be completed | This status code is returned when a message is received with a Require-Recipient-Valid-Since field or RRVS extension and the receiving system cannot complete the requested evaluation because the required timestamp was not recorded. The message originator needs to decide whether to reissue the message without RRVS protection. | RFC 7293 |
| SMTP error X.7.5 | Cryptographic failure | A transport system otherwise authorized to validate or decrypt a message in transport was unable to do so because necessary information such as key was not available or such information was invalid. | RFC 3463 |
| SMTP error X.1.4 | Destination mailbox address ambiguous | The mailbox address as specified matches one or more recipients on the destination system. This may result if a heuristic address mapping algorithm is used to map the specified address to a local mailbox name. | RFC 3463 |
| SMTP error X.1.0 | Other address status | Something about the address specified in the message caused this DSN. | RFC 3463 |
| SMTP error X.0.0 | Other undefined Status | Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known. | RFC 3463 |
| SMTP error 2.6.4 | Conversion with loss performed | This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message. | RFC 3463 |
| SMTP error 2.3.6 | Requested priority was changed | The message was accepted for relay/delivery, but the requested priority (possibly the implied default) was not honoured. The human readable text after the status code contains the new priority, followed by SP (space) and explanatory human readable text. | RFC 6710 |
| SMTP error 5.7.3 | Security conversion required but not possible | A conversion from one secure messaging protocol to another was required for delivery and such conversion was not possible. This is useful only as a permanent error. | RFC 3463 |
| SMTP error 5.7.15 | Priority Level is too low | The specified priority level is below the lowest priority acceptable for the receiving SMTP server. This condition might be temporary, for example the server is operating in a mode where only higher priority messages are accepted for transfer and delivery, while lower priority messages are rejected. | RFC 6710 |
| SMTP error 5.6.9 | UTF-8 header message cannot be transferred to one or more recipients, so the message must be rejected | This indicates that transaction failed after the final "." of the DATA command. | RFC 6531 |
| SMTP error 5.6.8 | UTF-8 string reply is required, but not permitted by the SMTP client | This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the SMTP client. | RFC 6531 |
| SMTP error 5.6.6 | Message content not available | The message content could not be fetched from a remote system. This may be useful as a permanent or persistent temporary notification. | RFC 4468 |
| SMTP error 5.6.5 | Conversion Failed | A conversion was required but was unsuccessful. This may be useful as a permanent or persistent temporary notification. | RFC 3463 |
| SMTP error 5.6.3 | Conversion required but not supported | The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path. This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop. | RFC 3463 |
| SMTP error 5.6.1 | Media not supported | The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error. | RFC 3463 |
| SMTP error 5.5.6 | Authentication Exchange line is too long | This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client sending a [BASE64] response which is longer than the maximum buffer size available for the currently selected SASL mechanism. This is useful for both permanent and persistent transient errors. | RFC 4954 |
| SMTP error 5.5.4 | Invalid command arguments | A valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. This is useful only as a permanent error. | RFC 3463 |
| SMTP error 5.5.0 | Other or undefined protocol status | Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes. | RFC 3463 |
Source. Codes, official sample texts, associated reply codes and definitions are taken from the IANA SMTP Enhanced Status Codes registry, which reproduces the defining RFC for each code. The leading class digit is set by the reporting server rather than by the registry: RFC 5321 section 4.2.1 fixes 4xx as a temporary failure and 5xx as a permanent one, so where IANA names an associated reply code the class follows from it. Where it does not, we say so rather than assign a digit. Causes and fixes are our own, from supporting UK business email.
Troubleshooting guides
This page explains the code. These guides walk through the fix in detail.