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+Mailbox sharing between users
+=============================
+
+To enable mailbox sharing, you'll need to create a shared namespace. See
+<ACL.txt> for more information about ACL-specific settings.
+
+---%<-------------------------------------------------------------------------
+# User's private mail location.
+mail_location = maildir:~/Maildir
+
+# When creating any namespaces, you must also have a private namespace:
+namespace {
+  type = private
+  separator = /
+  prefix =
+  #location defaults to mail_location.
+  inbox = yes
+}
+
+namespace {
+  type = shared
+  separator = /
+  prefix = shared/%%u/
+  # a) Per-user seen flags. Maildir indexes are shared. (INDEXPVT requires
+v2.2+)
+  location = maildir:%%h/Maildir:INDEXPVT=~/Maildir/shared/%%u
+  # b) Per-user seen flags. Maildir indexes are not shared. If users have
+direct filesystem level access to their mails, this is a safer option:
+  #location =
+maildir:%%h/Maildir:INDEX=~/Maildir/shared/%%u:INDEXPVT=~/Maildir/shared/%%u
+  subscriptions = no
+  list = children
+}
+
+mail_plugins = acl
+protocol imap {
+  mail_plugins = $mail_plugins imap_acl
+}
+plugin {
+  acl = vfile
+}
+---%<-------------------------------------------------------------------------
+
+This creates a shared/ namespace under which each user's mailboxes are. If you
+have multiple domains and allow sharing between them, you might want to set
+'prefix=shared/%%d/%%n/' instead (although %%u works just fine too). If you
+don't, you might want to drop the domain part and instead use
+'prefix=shared/%%n/'.
+
+'list=children' specifies that if no one has shared mailboxes to the user, the
+"shared" directory isn't listed by the LIST command. If you wish it to be
+visible always, you can set 'list=yes'.
+
+The 'location' setting specifies how to access other users' mailboxes. If you
+use %%h, the user's home directory is asked from auth process via auth-userdb
+socket. See <LDA.txt> for how to configure the socket. If the users' mailboxes
+can be found using a template, it's faster not to use the %%h. For example:
+
+---%<-------------------------------------------------------------------------
+  location = maildir:/var/mail/%%d/%%n/Maildir:INDEXPVT=~/Maildir/shared/%%u
+---%<-------------------------------------------------------------------------
+
+% vs %%
+-------
+
+%var expands to the logged in user's variable, while %%var expands to the other
+users' variables. For example if your name is "myself" and "someone1" and
+"someone2" have shared mailboxes to you, the variables could be expanded like:
+
+ * %u expands to "myself"
+ * %%u expands to "someone1" or "someone2"
+ * %h might expand to "/home/myself"
+ * %%h might expand to "/home/someone1" or "/home/someone2"
+ * ~/ equals %h/
+
+Note that in e.g. mail_location setting you might need both. For example in:
+
+---%<-------------------------------------------------------------------------
+mail_location = maildir:%%h/Maildir:INDEXPVT=%h/Maildir/shared/%%u
+---%<-------------------------------------------------------------------------
+
+What it means is:
+
+ * %%h/Maildir points to the other user's Maildir, e.g. "/home/someone1".
+ * :INDEXPVT=%h/Maildir/shared/%%u points to a per-user directory under your
+   own Maildir, e.g. "/home/myself/Maildir/someone1" or
+   "/home/myself/Maildir/someone2". This is necessary for storing per-user seen
+   flags.
+
+dbox
+----
+
+With dbox the index files are a very important part of the mailboxes. You must
+not try to :INDEX= to have copies of index files. This will only result in
+mailbox corruption. (INDEXPVT can be used though.)
+
+Filesystem permissions
+----------------------
+
+Dovecot assumes that it can access the other users' mailboxes. If you use
+multiple UNIX UIDs, you may have problems setting up the permissions so that
+the mailbox sharing works. Dovecot never modifies existing files' permissions.
+See <SharedMailboxes.Permissions.txt> for more information.
+
+Shared mailbox listing
+----------------------
+
+With the above configuration it's possible to open shared mailboxes if you know
+their name, but they won't be visible in the mailbox list. This is because
+Dovecot has no way of knowing what users have shared mailboxes to whom.
+Iterating through all users and looking inside their mail directories would be
+horribly inefficient for more than a couple users.
+
+To overcome this problem Dovecot needs a dictionary, which contains the list of
+users who have shared mailboxes and to whom they have shared. If the users
+aren't properly listed in this dictionary, their shared mailboxes won't be
+visible. Currently there's no way to automatically rebuild this dictionary, so
+make sure it doesn't get lost. If it does, each user having shared mailboxes
+must use the IMAP SETACL command (see below) to get the dictionary updated for
+themselves.
+
+You could use any dictionary backend, including SQL, but a simple flat file
+should work pretty well too:
+
+---%<-------------------------------------------------------------------------
+plugin {
+  acl_shared_dict = file:/var/lib/dovecot/db/shared-mailboxes.db
+}
+---%<-------------------------------------------------------------------------
+
+The IMAP processes must be able to write to the 'db/' directory. If you're
+using system users, you probably want to make it mode 0770 and group
+'sharedusers' and set 'mail_access_groups=sharedusers' (or something similar).
+
+If you use multiple domains and don't wish users to share their mailboxes to
+users in other domains, you can use separate dict files for each domain:
+
+---%<-------------------------------------------------------------------------
+plugin {
+  # assumes mailboxes are in /var/mail/%d/%n:
+  acl_shared_dict = file:/var/mail/%d/shared-mailboxes.db
+}
+---%<-------------------------------------------------------------------------
+
+Using SQL dictionary
+--------------------
+
+'dovecot.conf':
+
+---%<-------------------------------------------------------------------------
+plugin {
+  acl_shared_dict = proxy::acl
+}
+
+dict {
+  acl = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext
+}
+---%<-------------------------------------------------------------------------
+
+See <Dict.txt> for more information, especially about permission issues.
+
+Database tables:
+
+---%<-------------------------------------------------------------------------
+CREATE TABLE user_shares (
+  from_user varchar(100) not null,
+  to_user varchar(100) not null,
+  dummy char(1) DEFAULT '1',    -- always '1' currently
+  primary key (from_user, to_user)
+);
+COMMENT ON TABLE user_shares IS 'User from_user shares folders to user
+to_user.';
+
+CREATE INDEX to_user
+ON user_shares (to_user); -- because we always search for to_user
+
+CREATE TABLE anyone_shares (
+  from_user varchar(100) not null,
+  dummy char(1) DEFAULT '1',    -- always '1' currently
+  primary key (from_user)
+);
+COMMENT ON TABLE anyone_shares IS 'User from_user shares folders to anyone.';
+---%<-------------------------------------------------------------------------
+
+'/etc/dovecot/dovecot-dict-sql.conf.ext':
+
+---%<-------------------------------------------------------------------------
+connect = host=localhost dbname=mails user=sqluser password=sqlpass
+map {
+  pattern = shared/shared-boxes/user/$to/$from
+  table = user_shares
+  value_field = dummy
+
+  fields {
+    from_user = $from
+    to_user = $to
+  }
+}
+
+map {
+  pattern = shared/shared-boxes/anyone/$from
+  table = anyone_shares
+  value_field = dummy
+
+  fields {
+    from_user = $from
+  }
+}
+---%<-------------------------------------------------------------------------
+
+Mailbox sharing
+---------------
+
+You can use <doveadm acl> [Tools.Doveadm.ACL.txt] to share mailboxes or it can
+be done using IMAP SETACL command. It is the only way to update the shared
+mailbox list dictionary.
+
+Below is a quick introduction to IMAP ACL commands. See RFC 4314
+[http://www.ietf.org/rfc/rfc4314.txt] for more details.
+
+ * MYRIGHTS <mailbox>: Returns the user's current rights to the mailbox.
+ * GETACL <mailbox>: Returns the mailbox's all ACLs.
+ * SETACL <mailbox> <id> [+|-]<rights>: Give <id> the specified rights to the
+   mailbox.
+ * DELETEACL <mailbox> [-]<id>: Delete <id>'s ACL from the mailbox.
+
+<id> is one of:
+
+ * anyone: Matches all users, including anonymous users.
+ * authenticated: Like "anyone", but doesn't match anonymous users.
+ * $group: Matches all users belonging to the group ($ is not part of the group
+   name).
+ * $!group: See group-override in <ACL.txt> (Dovecot-specific feature).
+ * user: Matches the given user.
+
+The $group syntax is not a standard, but it is mentioned in RFC 4314 examples
+and is also understood by at least Cyrus IMAP. Having '-' before the identifier
+specifies negative rights.
+
+See <ACL.txt> for list of <rights>.
+
+Sharing mailboxes to everyone
+-----------------------------
+
+By default Dovecot doesn't allow using the IMAP "anyone" or "authenticated"
+identifier, because it would be an easy way to spam other users in the system.
+If you wish to allow it, set:
+
+---%<-------------------------------------------------------------------------
+plugin {
+  acl_anyone = allow
+}
+---%<-------------------------------------------------------------------------
+
+Note that you can also do this only for some users by using the second table
+"anyone_shares". Every user listed in this table shares his folders with
+everyone. See also <userdb extra field> [UserDatabase.ExtraFields.txt].
+
+IMAP ACL examples
+-----------------
+
+Let's begin with some simple example that first gives "read" and "lookup"
+rights, and later adds "write-seen" right:
+
+---%<-------------------------------------------------------------------------
+1 SETACL Work user@domain rl
+1 OK Setacl complete.
+
+2 SETACL Work user@domain +s
+2 OK Setacl complete.
+
+3 GETACL Work
+* ACL "Work" "user@domain" lrs "myself" lrwstipekxacd
+3 OK Getacl completed.
+---%<-------------------------------------------------------------------------
+
+Let's see how negative rights work by testing it on ourself. See how we
+initially have "lookup" right, but later we don't:
+
+---%<-------------------------------------------------------------------------
+1 MYRIGHTS Work
+* MYRIGHTS "Work" lrwstipekxacd
+1 OK Myrights completed.
+
+2 SETACL Work -myself l
+2 OK Setacl complete.
+
+3 GETACL Work
+* ACL "Work" "-myself" l "user@domain" lr "myself" lrwstipekxacd
+3 OK Getacl completed.
+
+4 myrights Work
+* MYRIGHTS "Work" rwstipekxacd
+4 OK Myrights completed.
+---%<-------------------------------------------------------------------------
+
+Troubleshooting
+---------------
+
+ * Make sure the % and %% variables are specified correctly in the namespace
+   location.'mail_debug=yes' will help you see if Dovecot is trying to access
+   correct paths.
+ * 'doveadm acl debug -u user@domain shared/user/box' can be helpful in
+   figuring out why a mailbox can't be accessed.
+
+(This file was created from the wiki on 2019-06-19 12:42)