imapsync performs stateful synchronization between two IMAP4rev1
servers.  Such synchronization is made possible by the QRESYNC extension
from [RFC7162]; for convenience reasons servers must also support
LIST-EXTENDED [RFC5258], LIST-STATUS [RFC5819] and UIDPLUS [RFC4315].
Furthermore, while imapsync can work with servers lacking support for
LITERAL+ [RFC2088] and MULTIAPPEND [RFC3502], these extensions greatly
improve performance by reducing the number of required round trips hence
are recommended.


Stateful synchronization is only possible for mailboxes supporting
persistent message Unique Identifiers (UID) and persistent storage of
mod-sequences (MODSEQ); any non-compliant mailbox will cause imapsync to
abort.  Furthermore, because UIDs are allocated not by the client but by
the server, imapsync needs to keep track of associations between local
and remote UIDs for each mailbox.  The synchronization state of a
mailbox consists of its UIDNEXT and HIGHESTMODSEQ values on each server;
it is then assumed that each message with UID < $UIDNEXT have been
replicated to the other server, and that the metadata (such as flags) of
each message with MODSEQ <= $HIGHESTMODSEQ have been synchronized.
Conceptually, the synchronization algorithm is derived from [RFC4549]
with the [RFC7162, section 6] amendments, and works as follows:


 1. SELECT (on both servers) a mailbox the current UIDNEXT or
    HIGHESTMODSEQ values of which differ from the values found in the
    database (for either server).  Use the QRESYNC SELECT parameter from
    [RFC7162] to list changes (vanished messages and flag updates) since
    $HIGHESTMODSEQ to messages with UID<$UIDNEXT.

 2. Propagate these changes onto the other server: get the corresponding
    UIDs from the database, then a/ issue an UID STORE + UID EXPUNGE
    command to remove messages that have not already been deleted on
    both servers, and b/ issue UID STORE commands to propagate flag
    updates (send a single command for each flag list in order the
    reduce the number of round trips).  (Conflicts may occur if the
    metadata of a message has been updated on both servers with
    different flag lists; in that case imapsync issues a warning and
    updates the message on each server with the union of both flag
    lists.)  Repeat this step if the server sent some updates in the
    meantime.  Otherwise, update the HIGHESTMODSEQ value in the
    database.

 3. Process new messages (if the current UIDNEXT value differ from the
    one found in the database) by issuing an UID FETCH command and for
    each message RFC822 body received, issue an APPEND command to the
    other server on-the-fly.  Repeat this step if the server received
    new messages in the meantime.  Otherwise, update the UIDNEXT value
    in the database.  Go back to step 2 if the server sent some updates
    in the meantime.

 4. Go back to step 1 to proceed with the next unsynchronized mailbox.


Consult the manual for more information.

imapsync is Copyright© 2015 Guilhem Moulin ⟨guilhem@fripost.org⟩, and
licensed for use under the GNU General Public License version 3 or
later.  See ‘COPYING’ for specific terms and distribution information.