Mbox Child Folders Configuration

Under mbox, it is not normally possible to have a mail folder which contains both messages and sub-folders. This is because there would be a filesystem name collision between the name of the mbox file containing the messages and the name of the directory containing the sub-folders. For example:

  • Mail folder “foo” containing messages would be stored in a file at ~/mail/foo.

  • Mail folder “foo/bar” containing messages would be stored in a file at ~/mail/foo/bar, but this cannot happen because this relies on the existence of a directory ~/mail/foo/ which can’t exist because there is already a file with that name.

Under mbox, Dovecot normally stores mail folders in “filesystem” layout. In this layout, mail folders are stored in mbox files (potentially under subdirectories) with the same relative path as the mail folder path. For example:

File

Description

~/mail/foo

mbox file containing mail for mail folder “foo”; cannot create any mail sub-folders of “foo”

~/mail/bar/baz

mbox file containing mail for mail folder “bar/baz”; cannot create any mail sub-folders of “bar/baz”

~/mail/inbox

mbox file containing mail for INBOX

If there is a requirement to be able to have a mail folder which contains both messages and sub-folders, then there are two ways to do it:

  1. Maildir++ layout

  2. Messages in named file

These approaches are described in more detail below.

Maildir++ Layout

Dovecot can be configured to keep mbox mail in a Maildir++-like layout. This makes Dovecot keep mail in mbox files where all the mailbox folder naming levels are separated with dots (with a leading dot). For example:

File

Description

~/mail/.foo

mbox file containing mail for mail folder “foo”

~/mail/.foo.bar

mbox file containing mail for mail folder “foo/bar”. We can now do this.

~/mail/.bar.baz

mbox file containing mail for mail folder “bar/baz”

~/mail/inbox

mbox file containing mail for INBOX

This can be enabled by adding :LAYOUT=maildir++ to the mail location:

# Incomplete example. Do not use!
mail_location = mbox:~/mail:LAYOUT=maildir++

However, there is a problem. Under mbox, setting LAYOUT=maildir++ alone leaves Dovecot unable to place index files, which would likely result in performance issues. So when using LAYOUT=maildir++` with mbox, it is advisable to also configure ``INDEX. Now, mail files (other than INBOX) all have names beginning with a dot, so if we like we can store other things in the ~/mail directory by using names which do not begin with a dot. So we could think to use INDEX to store indexes at ~/mail/index/. Example:

# Incomplete example. Do not use!
mail_location = mbox:~/mail:LAYOUT=maildir++:INDEX=~/mail/index

If we do this, then indexes will be kept at ~/mail/index/ and this will not clash with any names used for mail folders. There is one more thing we may want to consider though. By default Dovecot will maintain a list of subscribed folders in a file .subscriptions under the mail location root. In this case that means it would end up at ~/mail/.subscriptions. This would then mean that it would be impossible to create a mail folder called “subscriptions”. We can get around this by using the CONTROL parameter to move the .subscriptions file somewhere else, for example into the directory ~/mail/control (again, choosing a name which doesn’t begin with a dot so we don’t collide with the names of mbox files storing mail folders). That gives us:

# Trick mbox configuration which allows a mail folder which contains both
# messages and sub-folders
mail_location = mbox:~/mail:LAYOUT=maildir++:INDEX=~/mail/index:CONTROL=~/mail/control

This then allows mail folders which contains both messages and sub-folders without possibility of naming collisions between mail folders and other data.

There is one further wrinkle. Specifying :LAYOUT=maildir++ for mbox changes the default hierarchy separator from a slash to a dot. This should not be a problem for IMAP clients as the hierarchy separator is exposed through IMAP. However anything which expects to just “know” that the hierarchy separator is a slash may get confused. This can be worked around by configuring Namespaces to set the folder separator back to a slash.

Messages in Named File

In the default “filesystem” example from above, we can’t create any sub-folders of “foo” because there is a file - foo - in the way. So we could think to get rid of that file and put a directory there instead. But if we do that then we need somewhere to put the messages for folder “foo”. We could think to put them in a specially-named file in the directory: foo/. Then if we wanted to create a sub-folder of “foo” we would be fine because we could then do that. The rule would then be that messages go into the specially-named file in the directory corresponding to the mail folder name. We want want to choose a special name which would be unlikely to collide with a folder name. We could think to use something like mBoX-MeSsAgEs. Now, it turns out that you can configure Dovecot to do this using the DIRNAME parameter:

# Incomplete example. Do not use!
mail_location = mbox:~/mail:DIRNAME=mBoX-MeSsAgEs

With that config, we would get a layout like this:

File

Description

~/mail/inbox

mbox file containing mail for INBOX

~/mail/foo/mBoX-MeSsAgEs

mbox file containing mail for mail folder “foo”

~/mail/foo/bar/mBoX-MeSsAgEs

mbox file containing mail for mail folder “foo/bar”

However there is a problem. Under mbox, setting DIRNAME alone leaves Dovecot unable to place index files, which would likely result in performance issues, or worse, if the index directory gets created first, this will obstruct the creation of the mbox file. So when using DIRNAME with mbox, it is also necessary to configure INDEX. The question then arises where to put index files.

Any directory under the ~/mail directory could be considered as a mail folder. We could think to use a name beginning with a dot, for example ~/mail/.index but that would then mean that it would not be possible to create a mail folder called “.index”; unlikely, but it would be nice to have as few implementation-specific restrictions as possible.

In addition, by default, Dovecot will create a file .subscriptions at the mail location root to hold a list of mailbox subscriptions. This would make it impossible to create a mail folder called “.subscriptions”. But we can move the .subscriptions file to another directory by using the CONTROL parameter. To get around these issues, we can add another directory layer which separates these purposes. For example:

# Trick mbox configuration which allows a mail folder which contains both
# messages and sub-folders
mail_location = mbox:~/mail/mailboxes:DIRNAME=mBoX-MeSsAgEs:INDEX=~/mail/index:CONTROL=~/mail/control

would result in the following layout:

File

Description

~/mail/mailboxes/foo/mBoX-MeSsAgEs

mbox file containing messages for mail folder “foo”

~/mail/mailboxes/foo/bar/mBoX-MeSsAgEs

mbox file containing messages for mail folder “foo/bar”

~/mail/mailboxes/inbox

mbox file containing messages for INBOX

~/mail/control/.subscriptions

File containg list of subscribed mailboxes

~/mail/index/INBOX/dovecot.index.*

Index files for INBOX

~/mail/index/foo/dovecot.index.*

Index files for mail folder “foo”

~/mail/index/foo/bar/dovecot.index.*

Index files for mail folder “foo/bar”

~/mail/index/dovecot.mailbox.log

Other index files

Restrictions on mail folder names are then minimised; we can’t have mail folders with the names “mBoX-MeSsAgEs”, “dovecot.index.*, or “dovecot.mailbox.log”.

Unlike the Maildir++ layout approach above, because we are still using “filesystem” layout, the hierarchy separator remains as a slash.