7. Server Responses
Server responses are in three forms: status responses, server data,
and command continuation request. The information contained in a
server response, identified by "Contents:" in the response
descriptions below, is described by function, not by syntax. The
precise syntax of server responses is described in the Formal Syntax
section.
The client MUST be prepared to accept any response at all times.
Status responses can be tagged or untagged. Tagged status responses
indicate the completion result (OK, NO, or BAD status) of a client
command, and have a tag matching the command.
Some status responses, and all server data, are untagged. An
untagged response is indicated by the token "*" instead of a tag.
Untagged status responses indicate server greeting, or server status
that does not indicate the completion of a command (for example, an
impending system shutdown alert). For historical reasons, untagged
server data responses are also called "unsolicited data", although
strictly speaking only unilateral server data is truly "unsolicited".
Certain server data MUST be recorded by the client when it is
received; this is noted in the description of that data. Such data
conveys critical information which affects the interpretation of all
subsequent commands and responses (e.g. updates reflecting the
creation or destruction of messages).
Other server data SHOULD be recorded for later reference; if the
client does not need to record the data, or if recording the data has
no obvious purpose (e.g. a SEARCH response when no SEARCH command is
in progress), the data SHOULD be ignored.
An example of unilateral untagged server data occurs when the IMAP
connection is in selected state. In selected state, the server
checks the mailbox for new messages as part of command execution.
Normally, this is part of the execution of every command; hence, a
NOOP command suffices to check for new messages. If new messages are
found, the server sends untagged EXISTS and RECENT responses
reflecting the new size of the mailbox. Server implementations that
offer multiple simultaneous access to the same mailbox SHOULD also
send appropriate unilateral untagged FETCH and EXPUNGE responses if
another agent changes the state of any message flags or expunges any
messages.
Command continuation request responses use the token "+" instead of a
tag. These responses are sent by the server to indicate acceptance
of an incomplete client command and readiness for the remainder of
the command.
7.1. Server Responses - Status Responses
Status responses are OK, NO, BAD, PREAUTH and BYE. OK, NO, and BAD
may be tagged or untagged. PREAUTH and BYE are always untagged.
Status responses MAY include an OPTIONAL "response code". A response
code consists of data inside square brackets in the form of an atom,
possibly followed by a space and arguments. The response code
contains additional information or status codes for client software
beyond the OK/NO/BAD condition, and are defined when there is a
specific action that a client can take based upon the additional
information.
The currently defined response codes are:
ALERT The human-readable text contains a special alert
that MUST be presented to the user in a fashion
that calls the user's attention to the message.
NEWNAME Followed by a mailbox name and a new mailbox name.
A SELECT or EXAMINE is failing because the target
mailbox name no longer exists because it was
renamed to the new mailbox name. This is a hint to
the client that the operation can succeed if the
SELECT or EXAMINE is reissued with the new mailbox
name.
PARSE The human-readable text represents an error in
parsing the [RFC-822] header or [MIME-IMB] headers
of a message in the mailbox.
PERMANENTFLAGS Followed by a parenthesized list of flags,
indicates which of the known flags that the client
can change permanently. Any flags that are in the
FLAGS untagged response, but not the PERMANENTFLAGS
list, can not be set permanently. If the client
attempts to STORE a flag that is not in the
PERMANENTFLAGS list, the server will either reject
it with a NO reply or store the state for the
remainder of the current session only. The
PERMANENTFLAGS list can also include the special
flag \*, which indicates that it is possible to
create new keywords by attempting to store those
flags in the mailbox.
READ-ONLY The mailbox is selected read-only, or its access
while selected has changed from read-write to
read-only.
READ-WRITE The mailbox is selected read-write, or its access
while selected has changed from read-only to
read-write.
TRYCREATE An APPEND or COPY attempt is failing because the
target mailbox does not exist (as opposed to some
other reason). This is a hint to the client that
the operation can succeed if the mailbox is first
created by the CREATE command.
UIDVALIDITY Followed by a decimal number, indicates the unique
identifier validity value.
UNSEEN Followed by a decimal number, indicates the number
of the first message without the \Seen flag set.
Additional response codes defined by particular client or server
implementations SHOULD be prefixed with an "X" until they are
added to a revision of this protocol. Client implementations
SHOULD ignore response codes that they do not recognize.
7.1.1. OK Response
Contents: OPTIONAL response code
human-readable text
The OK response indicates an information message from the server.
When tagged, it indicates successful completion of the associated
command. The human-readable text MAY be presented to the user as
an information message. The untagged form indicates an
information-only message; the nature of the information MAY be
indicated by a response code.
The untagged form is also used as one of three possible greetings
at connection startup. It indicates that the connection is not
yet authenticated and that a LOGIN command is needed.
Example: S: * OK IMAP4rev1 server ready
C: A001 LOGIN fred blurdybloop
S: * OK [ALERT] System shutdown in 10 minutes
S: A001 OK LOGIN Completed
7.1.2. NO Response
Contents: OPTIONAL response code
human-readable text
The NO response indicates an operational error message from the
server. When tagged, it indicates unsuccessful completion of the
associated command. The untagged form indicates a warning; the
command can still complete successfully. The human-readable text
describes the condition.
Example: C: A222 COPY 1:2 owatagusiam
S: * NO Disk is 98% full, please delete unnecessary data
S: A222 OK COPY completed
C: A223 COPY 3:200 blurdybloop
S: * NO Disk is 98% full, please delete unnecessary data
S: * NO Disk is 99% full, please delete unnecessary data
S: A223 NO COPY failed: disk is full
7.1.3. BAD Response
Contents: OPTIONAL response code
human-readable text
The BAD response indicates an error message from the server. When
tagged, it reports a protocol-level error in the client's command;
the tag indicates the command that caused the error. The untagged
form indicates a protocol-level error for which the associated
command can not be determined; it can also indicate an internal
server failure. The human-readable text describes the condition.
Example: C: ...very long command line...
S: * BAD Command line too long
C: ...empty line...
S: * BAD Empty command line
C: A443 EXPUNGE
S: * BAD Disk crash, attempting salvage to a new disk!
S: * OK Salvage successful, no data lost
S: A443 OK Expunge completed
7.1.4. PREAUTH Response
Contents: OPTIONAL response code
human-readable text
The PREAUTH response is always untagged, and is one of three
possible greetings at connection startup. It indicates that the
connection has already been authenticated by external means and
thus no LOGIN command is needed.
Example: S: * PREAUTH IMAP4rev1 server logged in as Smith
7.1.5. BYE Response
Contents: OPTIONAL response code
human-readable text
The BYE response is always untagged, and indicates that the server
is about to close the connection. The human-readable text MAY be
displayed to the user in a status report by the client. The BYE
response is sent under one of four conditions:
1) as part of a normal logout sequence. The server will close
the connection after sending the tagged OK response to the
LOGOUT command.
2) as a panic shutdown announcement. The server closes the
connection immediately.
3) as an announcement of an inactivity autologout. The server
closes the connection immediately.
4) as one of three possible greetings at connection startup,
indicating that the server is not willing to accept a
connection from this client. The server closes the
connection immediately.
The difference between a BYE that occurs as part of a normal
LOGOUT sequence (the first case) and a BYE that occurs because of
a failure (the other three cases) is that the connection closes
immediately in the failure case.
Example: S: * BYE Autologout; idle for too long
7.2. Server Responses - Server and Mailbox Status
These responses are always untagged. This is how server and mailbox
status data are transmitted from the server to the client. Many of
these responses typically result from a command with the same name.
7.2.1. CAPABILITY Response
Contents: capability listing
The CAPABILITY response occurs as a result of a CAPABILITY
command. The capability listing contains a space-separated
listing of capability names that the server supports. The
capability listing MUST include the atom "IMAP4rev1".
A capability name which begins with "AUTH=" indicates that the
server supports that particular authentication mechanism.
Other capability names indicate that the server supports an
extension, revision, or amendment to the IMAP4rev1 protocol.
Server responses MUST conform to this document until the client
issues a command that uses the associated capability.
Capability names MUST either begin with "X" or be standard or
standards-track IMAP4rev1 extensions, revisions, or amendments
registered with IANA. A server MUST NOT offer unregistered or
non-standard capability names, unless such names are prefixed with
an "X".
Client implementations SHOULD NOT require any capability name
other than "IMAP4rev1", and MUST ignore any unknown capability
names.
Example: S: * CAPABILITY IMAP4rev1 AUTH=KERBEROS_V4 XPIG-LATIN
7.2.2. LIST Response
Contents: name attributes
hierarchy delimiter
name
The LIST response occurs as a result of a LIST command. It
returns a single name that matches the LIST specification. There
can be multiple LIST responses for a single LIST command.
Four name attributes are defined:
\Noinferiors It is not possible for any child levels of
hierarchy to exist under this name; no child levels
exist now and none can be created in the future.
\Noselect It is not possible to use this name as a selectable
mailbox.
\Marked The mailbox has been marked "interesting" by the
server; the mailbox probably contains messages that
have been added since the last time the mailbox was
selected.
\Unmarked The mailbox does not contain any additional
messages since the last time the mailbox was
selected.
If it is not feasible for the server to determine whether the
mailbox is "interesting" or not, or if the name is a \Noselect
name, the server SHOULD NOT send either \Marked or \Unmarked.
The hierarchy delimiter is a character used to delimit levels of
hierarchy in a mailbox name. A client can use it to create child
mailboxes, and to search higher or lower levels of naming
hierarchy. All children of a top-level hierarchy node MUST use
the same separator character. A NIL hierarchy delimiter means
that no hierarchy exists; the name is a "flat" name.
The name represents an unambiguous left-to-right hierarchy, and
MUST be valid for use as a reference in LIST and LSUB commands.
Unless \Noselect is indicated, the name MUST also be valid as an
argument for commands, such as SELECT, that accept mailbox
names.
Example: S: * LIST (\Noselect) "/" ~/Mail/foo
7.2.3. LSUB Response
Contents: name attributes
hierarchy delimiter
name
The LSUB response occurs as a result of an LSUB command. It
returns a single name that matches the LSUB specification. There
can be multiple LSUB responses for a single LSUB command. The
data is identical in format to the LIST response.
Example: S: * LSUB () "." #news.comp.mail.misc
7.2.4. STATUS Response
Contents: name
status parenthesized list
The STATUS response occurs as a result of an STATUS command. It
returns the mailbox name that matches the STATUS specification and
the requested mailbox status information.
Example: S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292)
7.2.5. SEARCH Response
Contents: zero or more numbers
The SEARCH response occurs as a result of a SEARCH or UID SEARCH
command. The number(s) refer to those messages that match the
search criteria. For SEARCH, these are message sequence numbers;
for UID SEARCH, these are unique identifiers. Each number is
delimited by a space.
Example: S: * SEARCH 2 3 6
7.2.6. FLAGS Response
Contents: flag parenthesized list
The FLAGS response occurs as a result of a SELECT or EXAMINE
command. The flag parenthesized list identifies the flags (at a
minimum, the system-defined flags) that are applicable for this
mailbox. Flags other than the system flags can also exist,
depending on server implementation.
The update from the FLAGS response MUST be recorded by the client.
Example: S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
7.3. Server Responses - Mailbox Size
These responses are always untagged. This is how changes in the size
of the mailbox are trasnmitted from the server to the client.
Immediately following the "*" token is a number that represents a
message count.
7.3.1. EXISTS Response
Contents: none
The EXISTS response reports the number of messages in the mailbox.
This response occurs as a result of a SELECT or EXAMINE command,
and if the size of the mailbox changes (e.g. new mail).
The update from the EXISTS response MUST be recorded by the
client.
Example: S: * 23 EXISTS
7.3.2. RECENT Response
Contents: none
The RECENT response reports the number of messages with the
\Recent flag set. This response occurs as a result of a SELECT or
EXAMINE command, and if the size of the mailbox changes (e.g. new
mail).
Note: It is not guaranteed that the message sequence numbers of
recent messages will be a contiguous range of the highest n
messages in the mailbox (where n is the value reported by the
RECENT response). Examples of situations in which this is not
the case are: multiple clients having the same mailbox open
(the first session to be notified will see it as recent, others
will probably see it as non-recent), and when the mailbox is
re-ordered by a non-IMAP agent.
The only reliable way to identify recent messages is to look at
message flags to see which have the \Recent flag set, or to do
a SEARCH RECENT.
The update from the RECENT response MUST be recorded by the
client.
Example: S: * 5 RECENT
7.4. Server Responses - Message Status
These responses are always untagged. This is how message data are
transmitted from the server to the client, often as a result of a
command with the same name. Immediately following the "*" token is a
number that represents a message sequence number.
7.4.1. EXPUNGE Response
Contents: none
The EXPUNGE response reports that the specified message sequence
number has been permanently removed from the mailbox. The message
sequence number for each successive message in the mailbox is
immediately decremented by 1, and this decrement is reflected in
message sequence numbers in subsequent responses (including other
untagged EXPUNGE responses).
As a result of the immediate decrement rule, message sequence
numbers that appear in a set of successive EXPUNGE responses
depend upon whether the messages are removed starting from lower
numbers to higher numbers, or from higher numbers to lower
numbers. For example, if the last 5 messages in a 9-message
mailbox are expunged; a "lower to higher" server will send five
untagged EXPUNGE responses for message sequence number 5, whereas
a "higher to lower server" will send successive untagged EXPUNGE
responses for message sequence numbers 9, 8, 7, 6, and 5.
An EXPUNGE response MUST NOT be sent when no command is in
progress; nor while responding to a FETCH, STORE, or SEARCH
The update from the EXPUNGE response MUST be recorded by the
client.
Example: S: * 44 EXPUNGE
7.4.2. FETCH Response
Contents: message data
The FETCH response returns data about a message to the client.
The data are pairs of data item names and their values in
parentheses. This response occurs as the result of a FETCH or
STORE command, as well as by unilateral server decision (e.g. flag
updates).
The current data items are:
BODY A form of BODYSTRUCTURE without extension data.
BODY[]<>
A string expressing the body contents of the
specified section. The string SHOULD be
interpreted by the client according to the content
transfer encoding, body type, and subtype.
If the origin octet is specified, this string is a
substring of the entire body contents, starting at
that origin octet. This means that BODY[]<0> MAY
be truncated, but BODY[] is NEVER truncated.
8-bit textual data is permitted if a [CHARSET]
identifier is part of the body parameter
parenthesized list for this section. Note that
headers (part specifiers HEADER or MIME, or the
header portion of a MESSAGE/RFC822 part), MUST be
7-bit; 8-bit characters are not permitted in
headers. Note also that the blank line at the end
of the header is always included in header data.
Non-textual data such as binary data MUST be
transfer encoded into a textual form such as BASE64
prior to being sent to the client. To derive the
original binary data, the client MUST decode the
transfer encoded string.
BODYSTRUCTURE A parenthesized list that describes the [MIME-IMB]
body structure of a message. This is computed by
the server by parsing the [MIME-IMB] header fields,
defaulting various fields as necessary.
For example, a simple text message of 48 lines and
2279 octets can have a body structure of: ("TEXT"
"PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 2279
48)
Multiple parts are indicated by parenthesis
nesting. Instead of a body type as the first
element of the parenthesized list there is a nested
body. The second element of the parenthesized list
is the multipart subtype (mixed, digest, parallel,
alternative, etc.).
For example, a two part message consisting of a
text and a BASE645-encoded text attachment can have
a body structure of: (("TEXT" "PLAIN" ("CHARSET"
"US-ASCII") NIL NIL "7BIT" 1152 23)("TEXT" "PLAIN"
("CHARSET" "US-ASCII" "NAME" "cc.diff")
"<960723163407.20117h@cac.washington.edu>"
"Compiler diff" "BASE64" 4554 73) "MIXED"))
Extension data follows the multipart subtype.
Extension data is never returned with the BODY
fetch, but can be returned with a BODYSTRUCTURE
fetch. Extension data, if present, MUST be in the
defined order.
The extension data of a multipart body part are in
the following order:
body parameter parenthesized list
A parenthesized list of attribute/value pairs
[e.g. ("foo" "bar" "baz" "rag") where "bar" is
the value of "foo" and "rag" is the value of
"baz"] as defined in [MIME-IMB].
body disposition
A parenthesized list, consisting of a
disposition type string followed by a
parenthesized list of disposition
attribute/value pairs. The disposition type and
attribute names will be defined in a future
standards-track revision to [DISPOSITION].
body language
A string or parenthesized list giving the body
language value as defined in [LANGUAGE-TAGS].
Any following extension data are not yet defined in
this version of the protocol. Such extension data
can consist of zero or more NILs, strings, numbers,
or potentially nested parenthesized lists of such
data. Client implementations that do a
BODYSTRUCTURE fetch MUST be prepared to accept such
extension data. Server implementations MUST NOT
send such extension data until it has been defined
by a revision of this protocol.
The basic fields of a non-multipart body part are
in the following order:
body type
A string giving the content media type name as
defined in [MIME-IMB].
body subtype
A string giving the content subtype name as
defined in [MIME-IMB].
body parameter parenthesized list
A parenthesized list of attribute/value pairs
[e.g. ("foo" "bar" "baz" "rag") where "bar" is
the value of "foo" and "rag" is the value of
"baz"] as defined in [MIME-IMB].
body id
A string giving the content id as defined in
[MIME-IMB].
body description
A string giving the content description as
defined in [MIME-IMB].
body encoding
A string giving the content transfer encoding as
defined in [MIME-IMB].
body size
A number giving the size of the body in octets.
Note that this size is the size in its transfer
encoding and not the resulting size after any
decoding.
A body type of type MESSAGE and subtype RFC822
contains, immediately after the basic fields, the
envelope structure, body structure, and size in
text lines of the encapsulated message.
A body type of type TEXT contains, immediately
after the basic fields, the size of the body in
text lines. Note that this size is the size in its
content transfer encoding and not the resulting
size after any decoding.
Extension data follows the basic fields and the
type-specific fields listed above. Extension data
is never returned with the BODY fetch, but can be
returned with a BODYSTRUCTURE fetch. Extension
data, if present, MUST be in the defined order.
The extension data of a non-multipart body part are
in the following order:
body MD5
A string giving the body MD5 value as defined in
[MD5].
body disposition
A parenthesized list with the same content and
function as the body disposition for a multipart
body part.
body language
A string or parenthesized list giving the body
language value as defined in [LANGUAGE-TAGS].
Any following extension data are not yet defined in
this version of the protocol, and would be as
described above under multipart extension data.
ENVELOPE A parenthesized list that describes the envelope
structure of a message. This is computed by the
server by parsing the [RFC-822] header into the
component parts, defaulting various fields as
necessary.
The fields of the envelope structure are in the
following order: date, subject, from, sender,
reply-to, to, cc, bcc, in-reply-to, and message-id.
The date, subject, in-reply-to, and message-id
fields are strings. The from, sender, reply-to,
to, cc, and bcc fields are parenthesized lists of
address structures.
An address structure is a parenthesized list that
describes an electronic mail address. The fields
of an address structure are in the following order:
personal name, [SMTP] at-domain-list (source
route), mailbox name, and host name.
[RFC-822] group syntax is indicated by a special
form of address structure in which the host name
field is NIL. If the mailbox name field is also
NIL, this is an end of group marker (semi-colon in
RFC 822 syntax). If the mailbox name field is
non-NIL, this is a start of group marker, and the
mailbox name field holds the group name phrase.
Any field of an envelope or address structure that
is not applicable is presented as NIL. Note that
the server MUST default the reply-to and sender
fields from the from field; a client is not
expected to know to do this.
FLAGS A parenthesized list of flags that are set for this
message.
INTERNALDATE A string representing the internal date of the
message.
RFC822 Equivalent to BODY[].
RFC822.HEADER Equivalent to BODY.PEEK[HEADER].
RFC822.SIZE A number expressing the [RFC-822] size of the
message.
RFC822.TEXT Equivalent to BODY[TEXT].
UID A number expressing the unique identifier of the
message.
Example: S: * 23 FETCH (FLAGS (\Seen) RFC822.SIZE 44827)
7.5. Server Responses - Command Continuation Request
The command continuation request response is indicated by a "+" token
instead of a tag. This form of response indicates that the server is
ready to accept the continuation of a command from the client. The
remainder of this response is a line of text.
This response is used in the AUTHORIZATION command to transmit server
data to the client, and request additional client data. This
response is also used if an argument to any command is a literal.
The client is not permitted to send the octets of the literal unless
the server indicates that it expects it. This permits the server to
process commands and reject errors on a line-by-line basis. The
remainder of the command, including the CRLF that terminates a
command, follows the octets of the literal. If there are any
additional command arguments the literal octets are followed by a
space and those arguments.
Example: C: A001 LOGIN {11}
S: + Ready for additional command text
C: FRED FOOBAR {7}
S: + Ready for additional command text
C: fat man
S: A001 OK LOGIN completed
C: A044 BLURDYBLOOP {102856}
S: A044 BAD No such command as "BLURDYBLOOP"
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