|Title||IMAP4 Compatibility with IMAP2 and IMAP2bis
Network Working Group M. Crispin
Request for Comments: 1732 University of Washington
Category: Informational December 1994
IMAP4 COMPATIBILITY WITH IMAP2 AND IMAP2BIS
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
This is a summary of hints and recommendations to enable an IMAP4
implementation to interoperate with implementations that conform to
earlier specifications. None of these hints and recommendations are
required by the IMAP4 specification; implementors must decide for
themselves whether they want their implementation to fail if it
encounters old software.
IMAP4 has been designed to be upwards compatible with earlier
specifications. For the most part, IMAP4 facilities that were not in
earlier specifications should be invisible to clients unless the
client asks for the facility.
In some cases, older servers may support some of the capabilities
listed as being "new in IMAP4" as experimental extensions to the
IMAP2 protocol described in RFC 1176.
This information may not be complete; it reflects current knowledge
of server and client implementations as well as "folklore" acquired
in the evolution of the protocol.
IMAP4 client interoperability with old servers
In general, a client should be able to discover whether an IMAP2
server supports a facility by trial-and-error; if an attempt to use a
facility generates a BAD response, the client can assume that the
server does not support the facility.
A quick way to check whether a server implementation supports the
IMAP4 specification is to try the CAPABILITY command. An OK response
that includes the IMAP4 capability value indicates a server that
supports IMAP4; a BAD response or one without the IMAP4 capability
value indicates an older server.
The following is a list of facilities that are only in IMAP4, and
suggestions for how new clients might interoperate with old servers:
A BAD response to this command indicates that the server
implements IMAP2 (or IMAP2bis) and not IMAP4.
Use the LOGIN command.
LSUB and LIST commands
Try the RFC 1176 FIND command.
* in a sequence
Use the number of messages in the mailbox from the EXISTS
SEARCH extensions (character set, additional criteria)
Reformulate the search request using only the searching
options listed in search_old in the IMAP4 grammar. This may
entail doing multiple searches to achieve the desired
BODYSTRUCTURE fetch data item
Try to fetch the non-extensible BODY data item.
body section number 0
Fetch the entire message and extract the header.
RFC822.HEADER.LINES and RFC822.HEADER.LINES.NOT fetch data items
Use RFC822.HEADER and remove the unwanted information.
BODY.PEEK[section], RFC822.PEEK, and RFC822.TEXT.PEEK fetch data
items Use the corresponding non-PEEK versions and manually
clear the \Seen flag as necessary.
UID fetch data item and the UID commands
No equivalent capabilitity exists in older servers.
FLAGS.SILENT, +FLAGS.SILENT, and -FLAGS.SILENT store data items
Use the corresponding non-SILENT versions and ignore the
untagged FETCH responses which com eback.
The following IMAP4 facilities were introduced in the experimental
IMAP2bis revisions to RFC-1176, and may be present in a server that
does not support the CAPABILITY command:
CREATE, DELETE, and RENAME commands
To test whether these commands are present, try a CREATE
INBOX command. If the response is NO, these commands are
supported by the server. If the response is BAD, they are
not. Older servers without the CREATE capability may sup-
port implicit creation of a mailbox by a COPY command with a
non-existant name as the destination.
To test whether this command is present, try to append a
zero-length stream to a mailbox name that is known not to
exist (or at least, highly unlikely to exist) on the remote
SUBSCRIBE and UNSUBSCRIBE commands
Try the form of these commands with the optional MAILBOX
Use the SELECT command instead.
flags and internal date argument to APPEND command
Try the APPEND without any flag list and internal date argu-
BODY, BODY[section], and FULL fetch data items
Use RFC822.TEXT and ALL instead. Server does not support
Use the appropriate FETCH command and ignore the unwanted
IMAP4 client implementations must accept all responses and data for-
mats documented in the IMAP4 specification, including those labeled
as obsolete. This includes the COPY and STORE unsolicited responses
and the old format of dates and times. In particular, client imple-
mentations must not treat a date/time as a fixed format string; nor
may they assume that the time begins at a particular octet.
IMAP4 client implementations must not depend upon the presence of any
server extensions that are not in the base IMAP4 specification.
The experimental IMAP2bis version specified that the TRYCREATE spe-
cial information token is sent as a separate unsolicited OK response
instead of inside the NO response.
The FIND BBOARDS, FIND ALL.BBOARDS, and BBOARD commands of RFC 1176
are removed from IMAP4. There is no equivalent to the bboard com-
mands, which provided a separate namespace with implicit restrictions
on what may be done in that namespace.
Older server implementations may automatically create the destination
mailbox on COPY if that mailbox does not already exist. This was how
a new mailbox was created in older specifications. If the server
does not support the CREATE command (see above for how to test for
this), it will probably create a mailbox on COPY.
Older server implementations may not preserve flags or internal dates
on COPY. Some server implementations may not permit the preservation
of certain flags on COPY or their setting with APPEND as site policy.
IMAP4 server interoperability with old clients
In general, there should be no interoperation problem between a
server conforming to the IMAP4 specification and a well-written
client that conforms to an earlier specification. Known problems are
Poor wording in the description of the CHECK command in earlier
specifications implied that a CHECK command is the way to get the
current number of messages in the mailbox. This is incorrect. A
CHECK command does not necessarily result in an EXISTS response.
Clients must remember the most recent EXISTS value sent from the
server, and should not generate unnecessary CHECK commands.
An incompatibility exists with COPY in IMAP4. COPY in IMAP4
servers does not automatically create the destination mailbox if
that mailbox does not already exist. This may cause problems with
old clients that expect automatic mailbox creation in COPY.
The PREAUTH unsolicited response is new in IMAP4. It is highly
unlikely that an old client would ever see this response.
The format of dates and times has changed due to the impending end
of the century. Clients that fail to accept a four-digit year or
a signed four-digit timezone value will not work properly with
An incompatibility exists with the use of "\" in quoted strings.
This is best avoided by using literals instead of quoted strings
if "\" or <"> is embedded in the string.
Security issues are not discussed in this memo.
Mark R. Crispin
Networks and Distributed Computing, JE-30
University of Washington
Seattle, WA 98195
Phone: (206) 543-5762