Mailbox sharing between users¶

To enable mailbox sharing, you’ll need to create a shared namespace. See ACL 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 Dovecot LDA 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 change :INDEX= to a user-specific location. 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 Filesystem permissions (in shared mailboxes) 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.

See acl_shared_dict for plugin setting information.

You could use any dictionary backend, including SQL or Cassandra, 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 Dovecot Dictionaries 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
  }
}

IMAP ACL examples¶

Let’s begin with some simple example that first gives “read” and “lookup” rights, and later adds “write-seen” right:

SETACL Work user@domain rl
OK Setacl complete.

SETACL Work user@domain +s
OK Setacl complete.

GETACL Work
* ACL "Work" "user@domain" lrs "myself" lrwstipekxacd
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:

MYRIGHTS Work
* MYRIGHTS "Work" lrwstipekxacd
OK Myrights completed.

SETACL Work -myself l
OK Setacl complete.

GETACL Work
* ACL "Work" "-myself" l "user@domain" lr "myself" lrwstipekxacd
OK Getacl completed.

myrights Work
* MYRIGHTS "Work" rwstipekxacd
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.

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