RFC 2060
Internet Message Access Protocol - Version 4rev1
Network Working Group M. Crispin Request for Comments: 2060 University of Washington Obsoletes: 1730 December 1996 Category: Standards Track INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev1 permits manipulation of remote message folders, called "mailboxes", in a way that is functionally equivalent to local mailboxes. IMAP4rev1 also provides the capability for an offline client to resynchronize with the server (see also [IMAP-DISC]). IMAP4rev1 includes operations for creating, deleting, and renaming mailboxes; checking for new messages; permanently removing messages; setting and clearing flags; [RFC-822] and [MIME-IMB] parsing; searching; and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev1 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev1 supports a single server. A mechanism for accessing configuration information to support multiple IMAP4rev1 servers is discussed in [ACAP]. IMAP4rev1 does not specify a means of posting mail; this function is handled by a mail transfer protocol such as [SMTP]. IMAP4rev1 is designed to be upwards compatible from the [IMAP2] and unpublished IMAP2bis protocols. In the course of the evolution of IMAP4rev1, some aspects in the earlier protocol have become obsolete. Obsolete commands, responses, and data formats which an IMAP4rev1 implementation may encounter when used with an earlier implementation are described in [IMAP-OBSOLETE].
Other compatibility issues with IMAP2bis, the most common variant of the earlier protocol, are discussed in [IMAP-COMPAT]. A full discussion of compatibility issues with rare (and presumed extinct) variants of [IMAP2] is in [IMAP-HISTORICAL]; this document is primarily of historical interest. Table of Contents IMAP4rev1 Protocol Specification ................................. 4 1. How to Read This Document ................................ 4 1.1. Organization of This Document ............................ 4 1.2. Conventions Used in This Document ........................ 4 2. Protocol Overview ........................................ 5 2.1. Link Level ............................................... 5 2.2. Commands and Responses ................................... 6 2.2.1. Client Protocol Sender and Server Protocol Receiver ...... 6 2.2.2. Server Protocol Sender and Client Protocol Receiver ...... 7 2.3. Message Attributes ....................................... 7 2.3.1. Message Numbers .......................................... 7 2.3.1.1. Unique Identifier (UID) Message Attribute ........ 7 2.3.1.2. Message Sequence Number Message Attribute ........ 9 2.3.2. Flags Message Attribute ................................... 9 2.3.3. Internal Date Message Attribute .......................... 10 2.3.4. [RFC-822] Size Message Attribute ......................... 11 2.3.5. Envelope Structure Message Attribute ..................... 11 2.3.6. Body Structure Message Attribute ......................... 11 2.4. Message Texts ............................................ 11 3. State and Flow Diagram ................................... 11 3.1. Non-Authenticated State .................................. 11 3.2. Authenticated State ...................................... 11 3.3. Selected State ........................................... 12 3.4. Logout State ............................................. 12 4. Data Formats ............................................. 12 4.1. Atom ..................................................... 13 4.2. Number ................................................... 13 4.3. String .................................................... 13 4.3.1. 8-bit and Binary Strings ................................. 13 4.4. Parenthesized List ....................................... 14 4.5. NIL ...................................................... 14 5. Operational Considerations ............................... 14 5.1. Mailbox Naming ........................................... 14 5.1.1. Mailbox Hierarchy Naming ................................. 14 5.1.2. Mailbox Namespace Naming Convention ...................... 14 5.1.3. Mailbox International Naming Convention .................. 15 5.2. Mailbox Size and Message Status Updates .................. 16 5.3. Response when no Command in Progress ..................... 16 5.4. Autologout Timer ......................................... 16 5.5. Multiple Commands in Progress ............................ 17
6. Client Commands .......................................... 17 6.1. Client Commands - Any State .............................. 18 6.1.1. CAPABILITY Command ....................................... 18 6.1.2. NOOP Command ............................................. 19 6.1.3. LOGOUT Command ........................................... 20 6.2. Client Commands - Non-Authenticated State ................ 20 6.2.1. AUTHENTICATE Command ..................................... 21 6.2.2. LOGIN Command ............................................ 22 6.3. Client Commands - Authenticated State .................... 22 6.3.1. SELECT Command ........................................... 23 6.3.2. EXAMINE Command .......................................... 24 6.3.3. CREATE Command ........................................... 25 6.3.4. DELETE Command ........................................... 26 6.3.5. RENAME Command ........................................... 27 6.3.6. SUBSCRIBE Command ........................................ 29 6.3.7. UNSUBSCRIBE Command ...................................... 30 6.3.8. LIST Command ............................................. 30 6.3.9. LSUB Command ............................................. 32 6.3.10. STATUS Command ........................................... 33 6.3.11. APPEND Command ........................................... 34 6.4. Client Commands - Selected State ......................... 35 6.4.1. CHECK Command ............................................ 36 6.4.2. CLOSE Command ............................................ 36 6.4.3. EXPUNGE Command .......................................... 37 6.4.4. SEARCH Command ........................................... 37 6.4.5. FETCH Command ............................................ 41 6.4.6. STORE Command ............................................ 45 6.4.7. COPY Command ............................................. 46 6.4.8. UID Command .............................................. 47 6.5. Client Commands - Experimental/Expansion ................. 48 6.5.1. X<atom> Command .......................................... 48 7. Server Responses ......................................... 48 7.1. Server Responses - Status Responses ...................... 49 7.1.1. OK Response .............................................. 51 7.1.2. NO Response .............................................. 51 7.1.3. BAD Response ............................................. 52 7.1.4. PREAUTH Response ......................................... 52 7.1.5. BYE Response ............................................. 52 7.2. Server Responses - Server and Mailbox Status ............. 53 7.2.1. CAPABILITY Response ...................................... 53 7.2.2. LIST Response ............................................. 54 7.2.3. LSUB Response ............................................ 55 7.2.4 STATUS Response .......................................... 55 7.2.5. SEARCH Response .......................................... 55 7.2.6. FLAGS Response ........................................... 56 7.3. Server Responses - Mailbox Size .......................... 56 7.3.1. EXISTS Response .......................................... 56 7.3.2. RECENT Response .......................................... 57
7.4. Server Responses - Message Status ........................ 57 7.4.1. EXPUNGE Response ......................................... 57 7.4.2. FETCH Response ........................................... 58 7.5. Server Responses - Command Continuation Request .......... 63 8. Sample IMAP4rev1 connection .............................. 63 9. Formal Syntax ............................................ 64 10. Author's Note ............................................ 74 11. Security Considerations .................................. 74 12. Author's Address ......................................... 75 Appendices ....................................................... 76 A. References ............................................... 76 B. Changes from RFC 1730 .................................... 77 C. Key Word Index ........................................... 79 IMAP4rev1 Protocol Specification 1. How to Read This Document 1.1. Organization of This Document This document is written from the point of view of the implementor of an IMAP4rev1 client or server. Beyond the protocol overview in section 2, it is not optimized for someone trying to understand the operation of the protocol. The material in sections 3 through 5 provides the general context and definitions with which IMAP4rev1 operates. Sections 6, 7, and 9 describe the IMAP commands, responses, and syntax, respectively. The relationships among these are such that it is almost impossible to understand any of them separately. In particular, do not attempt to deduce command syntax from the command section alone; instead refer to the Formal Syntax section. 1.2. Conventions Used in This Document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. The following terms are used in this document to signify the requirements of this specification. 1) MUST, or the adjective REQUIRED, means that the definition is an absolute requirement of the specification. 2) MUST NOT that the definition is an absolute prohibition of the specification.
3) SHOULD means that there may exist valid reasons in particular
circumstances to ignore a particular item, but the full
implications MUST be understood and carefully weighed before
choosing a different course.
4) SHOULD NOT means that there may exist valid reasons in
particular circumstances when the particular behavior is
acceptable or even useful, but the full implications SHOULD be
understood and the case carefully weighed before implementing
any behavior described with this label.
5) MAY, or the adjective OPTIONAL, means that an item is truly
optional. One vendor may choose to include the item because a
particular marketplace requires it or because the vendor feels
that it enhances the product while another vendor may omit the
same item. An implementation which does not include a
particular option MUST be prepared to interoperate with another
implementation which does include the option.
"Can" is used instead of "may" when referring to a possible
circumstance or situation, as opposed to an optional facility of
the protocol.
"User" is used to refer to a human user, whereas "client" refers
to the software being run by the user.
"Connection" refers to the entire sequence of client/server
interaction from the initial establishment of the network
connection until its termination. "Session" refers to the
sequence of client/server interaction from the time that a mailbox
is selected (SELECT or EXAMINE command) until the time that
selection ends (SELECT or EXAMINE of another mailbox, CLOSE
command, or connection termination).
Characters are 7-bit US-ASCII unless otherwise specified. Other
character sets are indicated using a "CHARSET", as described in
[MIME-IMT] and defined in [CHARSET]. CHARSETs have important
additional semantics in addition to defining character set; refer
to these documents for more detail.
2. Protocol Overview
2.1. Link Level
The IMAP4rev1 protocol assumes a reliable data stream such as
provided by TCP. When TCP is used, an IMAP4rev1 server listens on
port 143.
2.2. Commands and Responses An IMAP4rev1 connection consists of the establishment of a client/server network connection, an initial greeting from the server, and client/server interactions. These client/server interactions consist of a client command, server data, and a server completion result response. All interactions transmitted by client and server are in the form of lines; that is, strings that end with a CRLF. The protocol receiver of an IMAP4rev1 client or server is either reading a line, or is reading a sequence of octets with a known count followed by a line. 2.2.1. Client Protocol Sender and Server Protocol Receiver The client command begins an operation. Each client command is prefixed with an identifier (typically a short alphanumeric string, e.g. A0001, A0002, etc.) called a "tag". A different tag is generated by the client for each command. There are two cases in which a line from the client does not represent a complete command. In one case, a command argument is quoted with an octet count (see the description of literal in String under Data Formats); in the other case, the command arguments require server feedback (see the AUTHENTICATE command). In either case, the server sends a command continuation request response if it is ready for the octets (if appropriate) and the remainder of the command. This response is prefixed with the token "+". Note: If, instead, the server detected an error in the command, it sends a BAD completion response with tag matching the command (as described below) to reject the command and prevent the client from sending any more of the command. It is also possible for the server to send a completion response for some other command (if multiple commands are in progress), or untagged data. In either case, the command continuation request is still pending; the client takes the appropriate action for the response, and reads another response from the server. In all cases, the client MUST send a complete command (including receiving all command continuation request responses and command continuations for the command) before initiating a new command. The protocol receiver of an IMAP4rev1 server reads a command line from the client, parses the command and its arguments, and transmits server data and a server command completion result response.
2.2.2. Server Protocol Sender and Client Protocol Receiver Data transmitted by the server to the client and status responses that do not indicate command completion are prefixed with the token "*", and are called untagged responses. Server data MAY be sent as a result of a client command, or MAY be sent unilaterally by the server. There is no syntactic difference between server data that resulted from a specific command and server data that were sent unilaterally. The server completion result response indicates the success or failure of the operation. It is tagged with the same tag as the client command which began the operation. Thus, if more than one command is in progress, the tag in a server completion response identifies the command to which the response applies. There are three possible server completion responses: OK (indicating success), NO (indicating failure), or BAD (indicating protocol error such as unrecognized command or command syntax error). The protocol receiver of an IMAP4rev1 client reads a response line from the server. It then takes action on the response based upon the first token of the response, which can be a tag, a "*", or a "+". A client MUST be prepared to accept any server response at all times. This includes server data that was not requested. Server data SHOULD be recorded, so that the client can reference its recorded copy rather than sending a command to the server to request the data. In the case of certain server data, the data MUST be recorded. This topic is discussed in greater detail in the Server Responses section. 2.3. Message Attributes In addition to message text, each message has several attributes associated with it. These attributes may be retrieved individually or in conjunction with other attributes or message texts. 2.3.1. Message Numbers Messages in IMAP4rev1 are accessed by one of two numbers; the unique identifier and the message sequence number. 2.3.1.1. Unique Identifier (UID) Message Attribute A 32-bit value assigned to each message, which when used with the unique identifier validity value (see below) forms a 64-bit value
that is permanently guaranteed not to refer to any other message in
the mailbox. Unique identifiers are assigned in a strictly ascending
fashion in the mailbox; as each message is added to the mailbox it is
assigned a higher UID than the message(s) which were added
previously.
Unlike message sequence numbers, unique identifiers are not
necessarily contiguous. Unique identifiers also persist across
sessions. This permits a client to resynchronize its state from a
previous session with the server (e.g. disconnected or offline access
clients); this is discussed further in [IMAP-DISC].
Associated with every mailbox is a unique identifier validity value,
which is sent in an UIDVALIDITY response code in an OK untagged
response at mailbox selection time. If unique identifiers from an
earlier session fail to persist to this session, the unique
identifier validity value MUST be greater than the one used in the
earlier session.
Note: Unique identifiers MUST be strictly ascending in the mailbox
at all times. If the physical message store is re-ordered by a
non-IMAP agent, this requires that the unique identifiers in the
mailbox be regenerated, since the former unique identifers are no
longer strictly ascending as a result of the re-ordering. Another
instance in which unique identifiers are regenerated is if the
message store has no mechanism to store unique identifiers.
Although this specification recognizes that this may be
unavoidable in certain server environments, it STRONGLY ENCOURAGES
message store implementation techniques that avoid this problem.
Another cause of non-persistance is if the mailbox is deleted and
a new mailbox with the same name is created at a later date, Since
the name is the same, a client may not know that this is a new
mailbox unless the unique identifier validity is different. A
good value to use for the unique identifier validity value is a
32-bit representation of the creation date/time of the mailbox.
It is alright to use a constant such as 1, but only if it
guaranteed that unique identifiers will never be reused, even in
the case of a mailbox being deleted (or renamed) and a new mailbox
by the same name created at some future time.
The unique identifier of a message MUST NOT change during the
session, and SHOULD NOT change between sessions. However, if it is
not possible to preserve the unique identifier of a message in a
subsequent session, each subsequent session MUST have a new unique
identifier validity value that is larger than any that was used
previously.
2.3.1.2. Message Sequence Number Message Attribute A relative position from 1 to the number of messages in the mailbox. This position MUST be ordered by ascending unique identifier. As each new message is added, it is assigned a message sequence number that is 1 higher than the number of messages in the mailbox before that new message was added. Message sequence numbers can be reassigned during the session. For example, when a message is permanently removed (expunged) from the mailbox, the message sequence number for all subsequent messages is decremented. Similarly, a new message can be assigned a message sequence number that was once held by some other message prior to an expunge. In addition to accessing messages by relative position in the mailbox, message sequence numbers can be used in mathematical calculations. For example, if an untagged "EXISTS 11" is received, and previously an untagged "8 EXISTS" was received, three new messages have arrived with message sequence numbers of 9, 10, and 11. Another example; if message 287 in a 523 message mailbox has UID 12345, there are exactly 286 messages which have lesser UIDs and 236 messages which have greater UIDs. 2.3.2. Flags Message Attribute A list of zero or more named tokens associated with the message. A flag is set by its addition to this list, and is cleared by its removal. There are two types of flags in IMAP4rev1. A flag of either type may be permanent or session-only. A system flag is a flag name that is pre-defined in this specification. All system flags begin with "\". Certain system flags (\Deleted and \Seen) have special semantics described elsewhere. The currently-defined system flags are: \Seen Message has been read \Answered Message has been answered \Flagged Message is "flagged" for urgent/special attention \Deleted Message is "deleted" for removal by later EXPUNGE \Draft Message has not completed composition (marked as a draft).
\Recent Message is "recently" arrived in this mailbox. This
session is the first session to have been notified
about this message; subsequent sessions will not see
\Recent set for this message. This flag can not be
altered by the client.
If it is not possible to determine whether or not
this session is the first session to be notified
about a message, then that message SHOULD be
considered recent.
If multiple connections have the same mailbox
selected simultaneously, it is undefined which of
these connections will see newly-arrives messages
with \Recent set and which will see it without
\Recent set.
A keyword is defined by the server implementation. Keywords do
not begin with "\". Servers MAY permit the client to define new
keywords in the mailbox (see the description of the
PERMANENTFLAGS response code for more information).
A flag may be permanent or session-only on a per-flag basis.
Permanent flags are those which the client can add or remove
from the message flags permanently; that is, subsequent sessions
will see any change in permanent flags. Changes to session
flags are valid only in that session.
Note: The \Recent system flag is a special case of a
session flag. \Recent can not be used as an argument in a
STORE command, and thus can not be changed at all.
2.3.3. Internal Date Message Attribute
The internal date and time of the message on the server. This is not
the date and time in the [RFC-822] header, but rather a date and time
which reflects when the message was received. In the case of
messages delivered via [SMTP], this SHOULD be the date and time of
final delivery of the message as defined by [SMTP]. In the case of
messages delivered by the IMAP4rev1 COPY command, this SHOULD be the
internal date and time of the source message. In the case of
messages delivered by the IMAP4rev1 APPEND command, this SHOULD be
the date and time as specified in the APPEND command description.
All other cases are implementation defined.
2.3.4. [RFC-822] Size Message Attribute The number of octets in the message, as expressed in [RFC-822] format. 2.3.5. Envelope Structure Message Attribute A parsed representation of the [RFC-822] envelope information (not to be confused with an [SMTP] envelope) of the message. 2.3.6. Body Structure Message Attribute A parsed representation of the [MIME-IMB] body structure information of the message. 2.4. Message Texts In addition to being able to fetch the full [RFC-822] text of a message, IMAP4rev1 permits the fetching of portions of the full message text. Specifically, it is possible to fetch the [RFC-822] message header, [RFC-822] message body, a [MIME-IMB] body part, or a [MIME-IMB] header. 3. State and Flow Diagram An IMAP4rev1 server is in one of four states. Most commands are valid in only certain states. It is a protocol error for the client to attempt a command while the command is in an inappropriate state. In this case, a server will respond with a BAD or NO (depending upon server implementation) command completion result. 3.1. Non-Authenticated State In non-authenticated state, the client MUST supply authentication credentials before most commands will be permitted. This state is entered when a connection starts unless the connection has been pre- authenticated. 3.2. Authenticated State In authenticated state, the client is authenticated and MUST select a mailbox to access before commands that affect messages will be permitted. This state is entered when a pre-authenticated connection starts, when acceptable authentication credentials have been provided, or after an error in selecting a mailbox.
3.3. Selected State In selected state, a mailbox has been selected to access. This state is entered when a mailbox has been successfully selected. 3.4. Logout State In logout state, the connection is being terminated, and the server will close the connection. This state can be entered as a result of a client request or by unilateral server decision. +--------------------------------------+ |initial connection and server greeting| +--------------------------------------+ || (1) || (2) || (3) VV || || +-----------------+ || || |non-authenticated| || || +-----------------+ || || || (7) || (4) || || || VV VV || || +----------------+ || || | authenticated |<=++ || || +----------------+ || || || || (7) || (5) || (6) || || || VV || || || || +--------+ || || || || |selected|==++ || || || +--------+ || || || || (7) || VV VV VV VV +--------------------------------------+ | logout and close connection | +--------------------------------------+ (1) connection without pre-authentication (OK greeting) (2) pre-authenticated connection (PREAUTH greeting) (3) rejected connection (BYE greeting) (4) successful LOGIN or AUTHENTICATE command (5) successful SELECT or EXAMINE command (6) CLOSE command, or failed SELECT or EXAMINE command (7) LOGOUT command, server shutdown, or connection closed 4. Data Formats IMAP4rev1 uses textual commands and responses. Data in IMAP4rev1 can be in one of several forms: atom, number, string, parenthesized list, or NIL.
4.1. Atom An atom consists of one or more non-special characters. 4.2. Number A number consists of one or more digit characters, and represents a numeric value. 4.3. String A string is in one of two forms: literal and quoted string. The literal form is the general form of string. The quoted string form is an alternative that avoids the overhead of processing a literal at the cost of limitations of characters that can be used in a quoted string. A literal is a sequence of zero or more octets (including CR and LF), prefix-quoted with an octet count in the form of an open brace ("{"), the number of octets, close brace ("}"), and CRLF. In the case of literals transmitted from server to client, the CRLF is immediately followed by the octet data. In the case of literals transmitted from client to server, the client MUST wait to receive a command continuation request (described later in this document) before sending the octet data (and the remainder of the command). A quoted string is a sequence of zero or more 7-bit characters, excluding CR and LF, with double quote (<">) characters at each end. The empty string is represented as either "" (a quoted string with zero characters between double quotes) or as {0} followed by CRLF (a literal with an octet count of 0). Note: Even if the octet count is 0, a client transmitting a literal MUST wait to receive a command continuation request. 4.3.1. 8-bit and Binary Strings 8-bit textual and binary mail is supported through the use of a [MIME-IMB] content transfer encoding. IMAP4rev1 implementations MAY transmit 8-bit or multi-octet characters in literals, but SHOULD do so only when the [CHARSET] is identified.
Although a BINARY body encoding is defined, unencoded binary strings are not permitted. A "binary string" is any string with NUL characters. Implementations MUST encode binary data into a textual form such as BASE64 before transmitting the data. A string with an excessive amount of CTL characters MAY also be considered to be binary. 4.4. Parenthesized List Data structures are represented as a "parenthesized list"; a sequence of data items, delimited by space, and bounded at each end by parentheses. A parenthesized list can contain other parenthesized lists, using multiple levels of parentheses to indicate nesting. The empty list is represented as () -- a parenthesized list with no members. 4.5. NIL The special atom "NIL" represents the non-existence of a particular data item that is represented as a string or parenthesized list, as distinct from the empty string "" or the empty parenthesized list (). 5. Operational Considerations 5.1. Mailbox Naming The interpretation of mailbox names is implementation-dependent. However, the case-insensitive mailbox name INBOX is a special name reserved to mean "the primary mailbox for this user on this server". 5.1.1. Mailbox Hierarchy Naming If it is desired to export hierarchical mailbox names, mailbox names MUST be left-to-right hierarchical using a single character to separate levels of hierarchy. The same hierarchy separator character is used for all levels of hierarchy within a single name. 5.1.2. Mailbox Namespace Naming Convention By convention, the first hierarchical element of any mailbox name which begins with "#" identifies the "namespace" of the remainder of the name. This makes it possible to disambiguate between different types of mailbox stores, each of which have their own namespaces.
For example, implementations which offer access to USENET
newsgroups MAY use the "#news" namespace to partition the USENET
newsgroup namespace from that of other mailboxes. Thus, the
comp.mail.misc newsgroup would have an mailbox name of
"#news.comp.mail.misc", and the name "comp.mail.misc" could refer
to a different object (e.g. a user's private mailbox).
5.1.3. Mailbox International Naming Convention
By convention, international mailbox names are specified using a
modified version of the UTF-7 encoding described in [UTF-7]. The
purpose of these modifications is to correct the following problems
with UTF-7:
1) UTF-7 uses the "+" character for shifting; this conflicts with
the common use of "+" in mailbox names, in particular USENET
newsgroup names.
2) UTF-7's encoding is BASE64 which uses the "/" character; this
conflicts with the use of "/" as a popular hierarchy delimiter.
3) UTF-7 prohibits the unencoded usage of "\"; this conflicts with
the use of "\" as a popular hierarchy delimiter.
4) UTF-7 prohibits the unencoded usage of "~"; this conflicts with
the use of "~" in some servers as a home directory indicator.
5) UTF-7 permits multiple alternate forms to represent the same
string; in particular, printable US-ASCII chararacters can be
represented in encoded form.
In modified UTF-7, printable US-ASCII characters except for "&"
represent themselves; that is, characters with octet values 0x20-0x25
and 0x27-0x7e. The character "&" (0x26) is represented by the two-
octet sequence "&-".
All other characters (octet values 0x00-0x1f, 0x7f-0xff, and all
Unicode 16-bit octets) are represented in modified BASE64, with a
further modification from [UTF-7] that "," is used instead of "/".
Modified BASE64 MUST NOT be used to represent any printing US-ASCII
character which can represent itself.
"&" is used to shift to modified BASE64 and "-" to shift back to US-
ASCII. All names start in US-ASCII, and MUST end in US-ASCII (that
is, a name that ends with a Unicode 16-bit octet MUST end with a "-
").
For example, here is a mailbox name which mixes English, Japanese,
and Chinese text: ~peter/mail/&ZeVnLIqe-/&U,BTFw-
5.2. Mailbox Size and Message Status Updates
At any time, a server can send data that the client did not request.
Sometimes, such behavior is REQUIRED. For example, agents other than
the server MAY add messages to the mailbox (e.g. new mail delivery),
change the flags of message in the mailbox (e.g. simultaneous access
to the same mailbox by multiple agents), or even remove messages from
the mailbox. A server MUST send mailbox size updates automatically
if a mailbox size change is observed during the processing of a
command. A server SHOULD send message flag updates automatically,
without requiring the client to request such updates explicitly.
Special rules exist for server notification of a client about the
removal of messages to prevent synchronization errors; see the
description of the EXPUNGE response for more detail.
Regardless of what implementation decisions a client makes on
remembering data from the server, a client implementation MUST record
mailbox size updates. It MUST NOT assume that any command after
initial mailbox selection will return the size of the mailbox.
5.3. Response when no Command in Progress
Server implementations are permitted to send an untagged response
(except for EXPUNGE) while there is no command in progress. Server
implementations that send such responses MUST deal with flow control
considerations. Specifically, they MUST either (1) verify that the
size of the data does not exceed the underlying transport's available
window size, or (2) use non-blocking writes.
5.4. Autologout Timer
If a server has an inactivity autologout timer, that timer MUST be of
at least 30 minutes' duration. The receipt of ANY command from the
client during that interval SHOULD suffice to reset the autologout
timer.
5.5. Multiple Commands in Progress The client MAY send another command without waiting for the completion result response of a command, subject to ambiguity rules (see below) and flow control constraints on the underlying data stream. Similarly, a server MAY begin processing another command before processing the current command to completion, subject to ambiguity rules. However, any command continuation request responses and command continuations MUST be negotiated before any subsequent command is initiated. The exception is if an ambiguity would result because of a command that would affect the results of other commands. Clients MUST NOT send multiple commands without waiting if an ambiguity would result. If the server detects a possible ambiguity, it MUST execute commands to completion in the order given by the client. The most obvious example of ambiguity is when a command would affect the results of another command; for example, a FETCH of a message's flags and a STORE of that same message's flags. A non-obvious ambiguity occurs with commands that permit an untagged EXPUNGE response (commands other than FETCH, STORE, and SEARCH), since an untagged EXPUNGE response can invalidate sequence numbers in a subsequent command. This is not a problem for FETCH, STORE, or SEARCH commands because servers are prohibited from sending EXPUNGE responses while any of those commands are in progress. Therefore, if the client sends any command other than FETCH, STORE, or SEARCH, it MUST wait for a response before sending a command with message sequence numbers. For example, the following non-waiting command sequences are invalid: FETCH + NOOP + STORE STORE + COPY + FETCH COPY + COPY CHECK + FETCH The following are examples of valid non-waiting command sequences: FETCH + STORE + SEARCH + CHECK STORE + COPY + EXPUNGE
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