If you have email tracking turned on in FLG then occasionally you may see an email marked as “Failed” as shown below.A “Failed” email is where we either couldn't connect to the recipient's mail server, the mail server rejected our message (sometimes after many retries), or we had “Dropped” the email. A “Dropped” email occurs when a previous attempt to deliver to the same email address has failed with a permanent failure error codeYou may see more information about the reason for the failure if you view the email and look at the “Status” field. It will either show the email status code or “Dropped”.MTP status codes can be classified as either a basic code or an enhanced code. A basic SMTP server response message will always be in the format of a 3 digit code (e.g. 550). An enhanced SMTP server status code will be in the format of 3 digits separated by decimal places (e.g. 5.1.7). You may see either or both of these formats as the status code after a failure depending on the information the receiving server returns to us. Below are some of the most common examples you will encounter.Basic SMTP Server Status CodesBasic status codes are defined in RFC 5321 as follows:CodeMeaning421The service is unavailable, try again later450The requested action was not taken because the user’s mailbox was unavailable451The message was not sent due to a server error452The command was aborted because there isn’t enough server storage455The server cannot deal with the command at this time500The server could not recognise the command due to a syntax error501A syntax error was encountered in command parameters or arguments502This command is not implemented503The server has encountered a bad sequence of commands504A command parameter is not implemented550User’s mailbox address is invalid551The recipient is not local to the server552The action was aborted due to exceeded storage allocation553The command was aborted because the mailbox name is invalid554The transaction failed for some unstated reason555MAIL FROM/RCPT TO parameters not recognized or not implementedEnhanced SMTP Server Status CodesEnhanced status codes are defined in RFC 3463, and use the syntax  .  . :: 2 indicates a success4 indicates a temporary delivery error. 5 indicates a permanent delivery error.:1: Addressing2: Mailbox (the recipient)3: Mail system (the destination mail system)4: Network and routing5: Mail delivery protocol7: Security or policyOV: A 1 to 3 digit number that further classifies the error.The following table contains the enhanced status codes that are returned for the most common message delivery failures. CodeDescriptionFurther informationX.0.0Other undefined StatusOther 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.X.1.0Other address statusSomething about the address specified in the message caused this DSN.X.1.1Bad destination mailbox addressThe mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the "@" sign is invalid.X.1.2Bad destination system addressThe destination system specified in the address does not exist or is incapable of accepting mail. For Internet mail names, this means the address portion to the right of the "@" is invalid for mail. X.1.3Bad destination mailbox address syntaxThe destination address was syntactically invalid. This can apply to any field in the address. X.1.4Destination mailbox address ambiguousThe 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.X.1.6Destination mailbox had moved, no forwarding addressThe mailbox address provided was at one time valid, but mail is no longer being accepted for that address.X.1.7Bad sender’s mailbox address syntaxThe sender's address was syntactically invalid. This can apply to any field in the address.X1.8Bad sender’s system addressThe sender's system specified in the address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the "@" is invalid for mail.X.1.9Message relayed to non-compliant mailerThe 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.X.1.10Recipient address has null MXThis status code is returned when the associated address is marked as invalid using a null MX.X.2.0Other or undefined mailbox statusThe mailbox exists, but something about the destination mailbox has caused the sending of this DSN.X.2.1Mailbox disabled, not accepting messagesThe mailbox exists, but is not accepting messages. This may be a permanent error if the mailbox will never be re-enabled or a transient error if the mailbox is only temporarily disabled.X.2.2Mailbox fullThe mailbox is full because the user has exceeded a per-mailbox administrative quota or physical capacity. The general semantics implies that the recipient can delete messages to make more space available.X.2.3Message length exceeds administrative limitA per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit.X.2.4Mailing list expansion problemThe mailbox is a mailing list address and the mailing list was unable to be expanded.X.3.0Other or undefined mail system statusThe destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.X.3.1Mail system fullMail system storage has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages.X.3.2System not accepting network messagesThe host on which the mailbox is resident is not accepting messages. Examples of such conditions include an immanent shutdown, excessive load, or system maintenance.X.3.3System not capable of selected featuresSelected 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.X.3.4Message too big for systemThe message is larger than per-message size limit. This limit may either be for physical or administrative reasons.X.3.5System incorrectly configuredThe system is not configured in a manner that will permit it to accept this message.X.4.0Other or undefined network or routing statusSomething 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.X.4.1No answer from hostThe outbound connection attempt was not answered, because either the remote system was busy, or was unable to take a call.X.4.2Bad connectionThe outbound connection was established, but was unable to complete the message transaction, either because of time-out, or inadequate connection quality.X.4.3Directory server failureThe network system was unable to forward the message, because a directory server was unavailable. This is useful only as a persistent transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error.X.4.4Unable to routeThe mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server.X.4.5Mail system congestionThe mail system was unable to deliver the message because the mail system was congested. X.4.6Routing loop detectedA routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user- forwarding loopX.4.7Delivery time expiredThe 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.X.5.0Other or undefined protocol statusSomething 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.X.5.1Invalid commandA mail transaction protocol command was issued which was either out of sequence or unsupported. X.5.2Syntax errorA mail transaction protocol command was issued which could not be interpreted, either because the syntax was wrong or the command is unrecognized. X.5.3Too many recipientsMore recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the segmentation of the message into two, the remainder of the recipients to be delivered on a subsequent delivery attempt. It is included in this list in the event that such segmentation is not possible.X.5.4Invalid command argumentsA valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. X.5.5Wrong protocol versionA protocol version mis-match existed which could not be automatically resolved by the communicating parties.X.5.6Authentication Exchange line is too longThis 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.X.6.0Other or undefined media errorSomething 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.X.6.1Media not supportedThe media of the message is not supported by either the delivery protocol or the next system in the forwarding path.X.6.2Conversion required and prohibitedThe 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.X.6.3Conversion required but not supportedThe 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.X.6.5Conversion failedA conversion was required but was unsuccessful.X.6.6Message content not availableThe message content could not be fetched from a remote system.X.6.7Non-ASCII addresses not permitted for that sender/recipient This indicates the reception of a MAIL or RCPT command that non-ASCII addresses are not permitted.X.6.8UTF-8 string reply is required, but not permitted by the SMTP clientThis 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.X.6.9UTF-8 header message cannot be transferred to one or more recipients, so the message must be rejectedThis indicates that the transaction failed after the final "." of the DATA command.X.7.0Other or undefined security statusSomething related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes. This status code may also be used when the condition cannot be further described because of security policies in force.X.7.1Delivery not authorised, message refusedThe sender is not authorized to send to the destination. This can be the result of per-host or per-recipient filtering.X.7.2Mailing list expansion prohibitedThe sender is not authorized to send a message to the intended mailing list.X.7.3Security conversion required but not possibleA conversion from one secure messaging protocol to another was required for delivery and such conversion was not possibleX.7.4Security features not supportedA message contained security features such as secure authentication that could not be supported on the delivery protocol.X.7.5Cryptographic failureA 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.X.7.6Cryptographic algorithm not supportedA transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.X.7.7Message integrity failureA transport system otherwise authorized to validate a message was unable to do so because the message was corrupted or altered.X.7.8Authentication credentials invalidThis response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).X.7.9Authentication mechanism is too weakThis response to the AUTH command indicates that the selected authentication mechanism is weaker than server policy permits for that user. The client SHOULD retry with a new authentication mechanism.X.7.10Encryption neededThis indicates that the external strong privacy layer is needed in order to use the requested authentication mechanism. This is primarily intended for use with clear text authentication mechanisms. A client which receives this may activate a security layer such as TLS prior to authenticating, or attempt to use a stronger mechanism.X.7.12A password transition is neededThis response to the AUTH command indicates that the user needs to transition to the selected authentication mechanism. This is typically done by authenticating once using the [PLAIN] authentication mechanism. The selected mechanism SHOULD then work for authentications in subsequent sessions.X.7.13User account disabledSometimes a system administrator will have to disable a user's account (e.g., due to lack of payment, abuse, evidence of a break-in attempt, etc). This error code occurs after a successful authentication to a disabled account. This informs the client that the failure is permanent until the user contacts their system administrator to get the account re-enabled. It differs from a generic authentication failure where the client's best option is to present the passphrase entry dialog in case the user simply mistyped their passphrase.X.7.14Trust relationship requiredThe submission server requires a configured trust relationship with a third-party server in order to access the message content. X.7.15Priority level is too lowThe 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.X.7.16Message is too big for the specified priorityThe message is too big for the specified priority. This condition might be temporary, for example the server is operating in a mode where only higher priority messages below a certain size are accepted for transfer and delivery.X.7.17Mailbox owner has changedThis 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.X.7.18Domain owner has changedThis 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.X.7.19RRVS test cannot be completedThis 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.X.7.20No passing DKIM signature foundThis status code is returned when a message did not contain any passing DKIM signatures. X.7.21No acceptable DKIM signature foundThis status code is returned when a message contains one or more passing DKIM signatures, but none are acceptable.X.7.22No valid author-matched DKIM signature foundThis status code is returned when a message contains one or more passing DKIM signatures, but none are acceptable because none have an identifier(s) that matches the author address(es) found in the From header field. This is a special case of X.7.21.X.7.23SPF validation failedThis status code is returned when a message completed an SPF check that produced a "fail" result, contrary to local policy requirements.X.7.24SPF validation errorThis status code is returned when evaluation of SPF relative to an arriving message resulted in an error.X.7.25Reverse DNS validation failedThis status code is returned when an SMTP client's IP address failed a reverse DNS validation check, contrary to local policy requirements.X.7.26Multiple authentication checks failedThis status code is returned when a message failed more than one message authentication check, contrary to local policy requirements. The particular mechanisms that failed are not specified.X.7.27Sender address has null MXThis status code is returned when the associated sender address has a null MX, and the SMTP receiver is configured to reject mail from such sender (e.g. because it could not return a DSN).