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mbox configuration
==================

See <MailboxFormat.mbox.txt> for a complete description of how Dovecot has
implemented mbox support.

Mail location
-------------

In many systems the user's mails are by default stored in '/var/mail/username'
file. This file is called INBOX in IMAP world. Since IMAP supports multiple
mailboxes, you'll need to have a directory for them as well. Usually '~/mail'
is a good choice for this. For installation such as this, the mail location is
specified with:

---%<-------------------------------------------------------------------------
# %u is replaced with the username that logs in
mail_location = mbox:~/mail:INBOX=/var/mail/%u
---%<-------------------------------------------------------------------------

It's in no way a requirement to have the INBOX in '/var/mail/' directory. In
fact this often just brings problems because Dovecot might not be able to write
dotlock files to the directory (see below). You can avoid this completely by
just keeping everything in '~/mail/':

---%<-------------------------------------------------------------------------
# INBOX exists in ~/mail/inbox
mail_location = mbox:~/mail
---%<-------------------------------------------------------------------------

Index files
-----------

See <MailLocation#indexfiles> [MailLocation.txt] for full explanation of how to
change the index path. For example:

---%<-------------------------------------------------------------------------
mail_location = mbox:~/mail:INBOX=/var/mail/%u:INDEX=/var/indexes/%u
---%<-------------------------------------------------------------------------

Locking
-------

Make sure that all software accessing the mboxes are using the same locking
methods in the same order. The order is important to prevent deadlocking. From
Dovecot's side you can change these from 'mbox_read_locks' and
'mbox_write_locks' settings. See <MboxLocking.txt> for more information.

/var/mail/ dotlocks
-------------------

Often mbox write locks include dotlock, which means that Dovecot needs to
create a new "<mbox>.lock" file to the directory where the mbox file exists. If
your INBOXes are in '/var/mail/' directory you may have to give Dovecot write
access to the directory. There are two ways the '/var/mail/' directory's
permissions have traditionally been set up:

 * World-writable with sticky bit set, allowing anyone to create new files but
   not overwrite or delete existing files owned by someone else (ie. same as
   /tmp). You can do this with 'chmod a+rwxt /var/mail'
 * Directory owned by a mail group and the directory set to group-writable
   (mode=0770, group=mail)

You can give Dovecot access to mail group by setting:

---%<-------------------------------------------------------------------------
mail_privileged_group = mail
---%<-------------------------------------------------------------------------

NOTE: With <LDA.txt> the 'mail_privileged_group' setting unfortunately doesn't
work, so you'll have to use the sticky bit, disable dotlocking completely or
use LMTP server instead.

/var/mail/* permissions
-----------------------

In some systems the '/var/mail/$USER' files have 0660 mode permissions. This
causes Dovecot to try to preserve the file's group, and if it doesn't have
permissions to do so, it'll fail with an error:

---%<-------------------------------------------------------------------------
imap(user): Error: chown(/home/user/mail/.imap/INBOX, -1, 12(mail)) failed:
Operation not permitted (egid=1000(user), group based on /var/mail/user)
---%<-------------------------------------------------------------------------

There is rarely any real need for the files to have 0660 mode, so the best
solution for this problem is to just change the mode to 0600:

---%<-------------------------------------------------------------------------
chmod 0600 /var/mail/*
---%<-------------------------------------------------------------------------

Optimizations
-------------

The settings below are related to mbox performance. See
<MailboxFormat.mbox.txt> for more complete description of what they do.

 * 'mbox_lazy_writes=yes' (default): Metadata updates, such as writing X-UID
   headers or flag changes, aren't written to mbox file until the mailbox is
   closed or CHECK or EXPUNGE IMAP commands are sent by the client. The mbox
   rewrites can be costly, so this may avoid a lot of disk writes.
 * 'mbox_dirty_syncs=yes' (default): Dovecot assumes that external mbox file
   changes only mean that new messages were appended to it. Without this
   setting Dovecot re-reads the whole mbox file whenever it changes. There are
   various safeguards in place to make this setting safe even when other
   changes than appends were done to the mbox. The only downside to this
   setting is that external message flag modifications may not be visible
   immediately.
 * 'mbox_very_dirty_syncs=yes' (not default): When opening mbox file that has
   been changed externally, don't re-read it. Otherwise similar to
   'mbox_dirty_syncs=yes'.
 * 'mbox_min_index_size=n': If mbox file is smaller than n kilobytes, don't
   update its index files. If an index file exists for it, it's still read
   however.

Only /var/mail/ mboxes
----------------------

With POP3 it's been traditional that users have their mails only in the
'/var/mail/' directory. IMAP however supports having multiple mailboxes, so
each user has to have a private directory where the mailboxes are stored.
Dovecot also needs a directory for its index files unless you disable them
completely.

If you *really* want to use Dovecot as a plain POP3 server without index files,
you can work around the problem of not having the per-user directory:

 * Set users' home directory in userdb to some empty non-writable directory,
   for example '/var/empty'
 * Modify 'mail_location' setting so that the mail root directory is also the
   empty directory and append ':INDEX=MEMORY' to it. For example:
   'mail_location = mbox:/var/empty:INBOX=/var/mail/%u:INDEX=MEMORY'
 * Note that if you have IMAP users, they'll see the '/var/empty' as the
   directory containing other mailboxes than INBOX. If the directory is
   writable, all the users will have their mailboxes shared.

Directory layout
----------------

By default Dovecot uses filesystem layout under mbox. This means that mail is
stored in mbox files under hierarchical directories, for example:

 * '~/mail/inbox' - mbox file containing mail for INBOX
 * '~/mail/foo' - mbox file containing mail for mailbox "foo"
 * '~/mail/bar/baz' - mbox file containing mail for mailbox "bar/baz"

One upshot of this is that it is not normally possible to have mailboxes which
are subfolders of mailboxes containing messages.

As an alternative, it is possible to configure Dovecot to store all mailboxes
in a single directory with hierarchical levels separated by a dot. This can be
configured by adding ':LAYOUT=maildir++' to the mail location. There are,
however, some further considerations when doing this; see
<MboxChildFolders.txt> for some examples.

Control files
-------------

Under mbox format, Dovecot maintains the subscribed mailboxes list in a file
'.subscriptions' which by default is stored in the mail location root. So in
the example configuration this would be at '~/mail/.subscriptions'.

If you want to put this somewhere else, you can change the directory in which
the '.subscriptions' file is kept by using the 'CONTROL' parameter. So for
example, if we configured the mail location using:

---%<-------------------------------------------------------------------------
mail_location = mbox:~/mail:CONTROL=~/mail-control
---%<-------------------------------------------------------------------------

then the subscribed mailboxes list would be maintained at
'~/mail-control/.subscriptions'.

One practical application of the 'CONTROL' parameter is described at
<MboxChildFolders.txt>.

Message file name
-----------------

By default, Dovecot stores messages for INBOX in an mbox file called "inbox",
and messages for all other mailboxes in an mbox file whose relative path is
equivalent to the name of the mailbox. Under this scheme, it is not possible to
have mailboxes which contain both messages and child mailboxes.

However, the behaviour (for mailboxes other than INBOX) can be changed using
the 'DIRNAME' parameter. If the 'DIRNAME' parameter is specified with a
particular value, then Dovecot will store messages in a file with a name of
that value, in a directory with a name equivalent to the mailbox name.

There are, however, some further considerations when doing this; see
<MboxChildFolders.txt> for an example.

(This file was created from the wiki on 2019-06-19 12:42)