IMAP4 Compatibility with IMAP2 and IMAP2bis
RFC 1732

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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.

Introduction

   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.

Crispin                                                         [Page 1]
RFC 1732                 IMAP4 - Compatibility             December 1994

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:

   CAPABILITY command
            A BAD response to this command indicates that the server
            implements IMAP2 (or IMAP2bis) and not IMAP4.

   AUTHENTICATE command.
            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
            unsolicited response.

   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
            results.

   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.

Crispin                                                         [Page 2]
RFC 1732                 IMAP4 - Compatibility             December 1994

   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.

   APPEND command
            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
            system.

   SUBSCRIBE and UNSUBSCRIBE commands
            Try the form of these commands with the optional MAILBOX