The leading character "X" is 4 for temporary errors and 5 for permanent ones. The SMTP Error Codes associated with the status code are listed.
X.0.0 Other undefined Status - Code: Any
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.
X.1.0 Other address status - Code: Not given
Something about the address specified in the message caused this DSN.
X.1.1 Bad destination mailbox address - Code: 451, 550
The 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. This code is only useful for permanent failures.
X.1.2 Bad destination system address - Code: Not given
The 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. This code is only useful for permanent failures.
X.1.3 Bad destination mailbox address syntax - Code: 501
The destination address was syntactically invalid. This can
apply to any field in the address. This code is only useful
for permanent failures.
X.1.4 Destination mailbox address ambiguous - Code: Not given
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.
X.1.6 Destination mailbox has moved, No forwarding address - Code: Not given
The mailbox address provided was at one time valid, but mail is
no longer being accepted for that address. This code is only
useful for permanent failures.
X.1.7 Bad sender's mailbox address syntax - Code: Not given
The sender's address was syntactically invalid. This can apply
to any field in the address.
X.1.8 Bad sender's system address - Code: 451, 501
The 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.1.9 Message relayed to non-compliant mailer - Code: Not given
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.
X.1.10 Recipient address has null MX - Code: 556
This status code is returned when the associated
address is marked as undeliverable using a null MX.
X.2.0 Other or undefined mailbox status - Code: Not given
The mailbox exists, but something about the destination mailbox
has caused the sending of this DSN.
X.2.1 Mailbox disabled, not accepting messages - Code: Not given
The 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.2.2 Mailbox full - Code: 552
The 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. This code should be used as a
persistent transient failure.
X.2.3 Message length exceeds administrative limit - Code: 552
A 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.
This code should be used as a permanent failure.
X.2.4 Mailing list expansion problem - Code: 450, 452
The mailbox is a mailing list address and the mailing list was
unable to be expanded. This code may represent a permanent
failure or a persistent transient failure.
X.3.0 Other or undefined mail system status - Code: 421, 451, 550, 554
The destination system exists and normally accepts mail, but
something about the system has caused the generation of this
DSN.
X.3.1 Mail system full - Code: 452
Mail 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. This is useful
only as a persistent transient error.
X.3.2 System not accepting network messages - Code: 453
The host on which the mailbox is resident is not accepting
messages. Examples of such conditions include an immanent
shutdown, excessive load, or system maintenance. This is
useful for both permanent and persistent transient errors.
X.3.3 System not capable of selected features - Code: Not given
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.
X.3.4 Message too big for system - Code: 552, 554
The message is larger than per-message size limit. This limit
may either be for physical or administrative reasons. This is
useful only as a permanent error.
X.3.5 System incorrectly configured - Code: Not given
The system is not configured in a manner that will permit it to
accept this message.
X.4.0 Other or undefined network or routing status - Code: Not given
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.
X.4.1 No answer from host - Code: 451
The outbound connection attempt was not answered, because
either the remote system was busy, or was unable to take a
call. This is useful only as a persistent transient error.
X.4.2 Bad connection - Code: 421
The outbound connection was established, but was unable to
complete the message transaction, either because of time-out,
or inadequate connection quality. This is useful only as a
persistent transient error.
X.4.3 Directory server failure - Code: 451, 550
The 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.4.4 Unable to route - Code: Not given
The mail system was unable to determine the next hop for the
message because the necessary routing information was
unavailable from the directory server. This is useful for both
permanent and persistent transient errors.
A DNS lookup returning only an SOA (Start of Administration)
record for a domain name is one example of the unable to route
error.
X.4.5 Mail system congestion - Code: 451
The mail system was unable to deliver the message because the
mail system was congested. This is useful only as a persistent
transient error.
X.4.6 Routing loop detected - Code: Not given
A routing loop caused the message to be forwarded too many
times, either because of incorrect routing tables or a user-
forwarding loop. This is useful only as a persistent transient
error.
X.4.7 Delivery time expired - Code: Not given
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.
X.5.0 Other or undefined protocol status - Code: 451, 452, 454, 458, 459, 501, 502, 503, 554
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.
X.5.1 Invalid command - Code: 430, 500, 501, 503, 530, 550, 554, 555
A mail transaction protocol command was issued which was either
out of sequence or unsupported. This is useful only as a
permanent error.
X.5.2 Syntax error - Code: 500, 501, 502, 550, 555
A mail transaction protocol command was issued which could not
be interpreted, either because the syntax was wrong or the
command is unrecognized. This is useful only as a permanent
error.
X.5.3 Too many recipients - Code: 451
More 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.5.4 Invalid command arguments - Code: 451, 501, 502, 503, 504, 550, 555
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.
X.5.5 Wrong protocol version - Code: Not given
A protocol version mis-match existed which could not be
automatically resolved by the communicating parties.
X.5.6 Authentication Exchange line is too long - Code: 500
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.
X.6.0 Other or undefined media error - Code: Not given
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.
X.6.1 Media not supported - Code: Not given
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.
X.6.2 Conversion required and prohibited - Code: Not given
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.
X.6.3 Conversion required but not supported - Code: 554
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.
X.6.5 Conversion Failed - Code: Not given
A conversion was required but was unsuccessful. This may be
useful as a permanent or persistent temporary notification.
X.6.6 Message content not available - Code: 554
The message content could not be fetched from a remote system.
This may be useful as a permanent or persistent temporary
notification.
X.6.7 Non-ASCII addresses not permitted for that sender/recipient - Code: 553, 550
This indicates the reception of a MAIL or RCPT
command that non-ASCII addresses are not permitted
X.6.8 UTF-8 string reply is required, but not permitted by the SMTP client - Code: 553, 550
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.
X.6.9 UTF-8 header message cannot be transferred to one or more recipients, so the message must be rejected - Code: 550
This indicates that transaction failed
after the final "." of the DATA command.
X.6.10 This is a duplicate of X.6.8 and is thus deprecated.
X.7.0 Other or undefined security status - Code: 450, 454, 500, 501, 503, 504, 530, 535, 550
Something 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.7.1 Delivery not authorized, message refused - Code: 451, 454, 502, 503, 533, 550, 551
The sender is not authorized to send to the destination. This
can be the result of per-host or per-recipient filtering. This
memo does not discuss the merits of any such filtering, but
provides a mechanism to report such. This is useful only as a
permanent error.
X.7.2 Mailing list expansion prohibited - Code: 550
The sender is not authorized to send a message to the intended
mailing list. This is useful only as a permanent error.
X.7.3 Security conversion required but not possible - Code: Not given
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.
X.7.4 Security features not supported - Code: 504
A message contained security features such as secure
authentication that could not be supported on the delivery
protocol. This is useful only as a permanent error.
X.7.5 Cryptographic failure - Code: Not given
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.
X.7.6 Cryptographic algorithm not supported - Code: Not given
A transport system otherwise authorized to validate or decrypt
a message was unable to do so because the necessary algorithm
was not supported.
X.7.7 Message integrity failure - Code: Not given
A transport system otherwise authorized to validate a message
was unable to do so because the message was corrupted or
altered. This may be useful as a permanent, transient
persistent, or successful delivery code.
X.7.8 Authentication credentials invalid - Code: 535, 554
This 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.7.9 Authentication mechanism is too weak - Code: 534
This 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.7.10 Encryption Needed - Code: 523
This indicates that 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.7.11 Encryption required for requested authentication mechanism - Code: 524, 538
This response to the AUTH command indicates that the selected
authentication mechanism may only be used when the underlying SMTP
connection is encrypted. Note that this response code is documented
here for historical purposes only. Modern implementations SHOULD NOT
advertise mechanisms that are not permitted due to lack of
encryption, unless an encryption layer of sufficient strength is
currently being employed.
X.7.12 A password transition is needed - Code: 422, 432
This 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.7.13 User Account Disabled - Code: 525
Sometimes 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.7.14 Trust relationship required - Code: 535, 554
The submission server requires a configured trust
relationship with a third-party server in order
to access the message content. This value
replaces the prior use of X.7.8 for this error
condition. thereby updating [RFC4468].
X.7.15 Priority Level is too low - Code: 450, 550 (other 4XX or 5XX codes are allowed)
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.
X.7.16 Message is too big for the specified priority - Code: 552 (other 4XX or 5XX codes are allowed)
The 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
certain size are accepted for transfer and delivery.
X.7.17 Mailbox owner has changed - Code: 5XX
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.
X.7.18 Domain owner has changed - Code: 5XX
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.
X.7.19 RRVS test cannot be completed - Code: 5XX
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.
X.7.20 No passing DKIM signature found - Code: 550
This status code is returned when a message
did not contain any passing DKIM signatures. (This violates the
advice of Section 6.1 of [RFC6376].)
X.7.21 No acceptable DKIM signature found - Code: 550
This status code is returned when a message
contains one or more passing DKIM signatures,
but none are acceptable. (This violates the
advice of Section 6.1 of [RFC6376].)
X.7.22 No valid author-matched DKIM signature found - Code: 550
This 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. (This violates the
advice of Section 6.1 of [RFC6376].)
X.7.23 SPF validation failed - Code: 550
This status code is returned when a message
completed an SPF check that produced a "fail" result, contrary to local policy
requirements. Used in place of 5.7.1 as described in Section 8.4 of
[RFC7208].
X.7.24 SPF validation error - Code: 451/550
This status code is returned when evaluation
of SPF relative to an arriving message resulted in an error. Used in place of
4.4.3 or 5.5.2 as described in Sections 8.6 and 8.7 of
[RFC7208].
X.7.25 Reverse DNS validation failed - Code: 550
This status code is returned when an SMTP
client's IP address failed a reverse DNS
validation check, contrary to local policy
requirements.
X.7.26 Multiple authentication checks failed - Code: 550
This 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.7.27 Sender address has null MX - Code: 550
This 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).