IMAP Configuration

Dovecot was optimized since the beginning to work as an efficient IMAP server.

Namespaces

See Namespaces.

IMAP Extensions

Dovecot supports many IMAP extensions.

COMPRESS

Dovecot supports the IMAP COMPRESS (RFC 4978) extension.

It allows an IMAP client to dynamically enable stream compression for an IMAP session.

The extension is enabled by default and configured with the default compression level for the available mechanism.

SEARCH=FUZZY

IMAP provides SEARCH as part of the core protocol, so it is useful to activate a Full Text Search indexing backend to handle these searches.

See fts plugin.

METADATA

Dovecot supports the IMAP METADATA extension (RFC 5464), which allows per-mailbox, per-user data to be stored and accessed via IMAP commands.

See imap_metadata for configuration instructions.

Storing Metadata in SQL Dictionary

You can store metadata into a database. This works best with a dedicated table for storing the entires.

SQL Schema

-- Since username is a primary key, it is required to have some value.
-- When empty, it means that the value applies to keys with 'shared/' prefix.
-- Keys with 'priv/' prefix are expected to have a non-empty username.

CREATE TABLE metadata (
  username VARCHAR(255) NOT NULL DEFAULT '',
  attr_name VARCHAR(255) NOT NULL,
  attr_value VARCHAR(65535),
  PRIMARY KEY(username, attr_name)
);

/etc/dovecot/dict.config

## driver specific config excluded

map {
  pattern = $key
  table = attr_priv
  fields {
    attr_name = $key
  }
  username_field = username
  value_field = attr_value
}

/etc/dovecot/dovecot.conf

dict {
  metadata = driver:/etc/dovecot/dict.config
}

mail_attribute_dict = proxy::metadata

SPECIAL-USE

⚠️ TODO

PREVIEW

Dovecot supports the PREVIEW extension (RFC 8970), retrieved via the IMAP FETCH command.

The extension is enabled by default. Preview text is generated during message delivery and is stored in the Dovecot index files.

NOTIFY

Set mailbox_list_index = yes.

URLAUTH:

Set imap_urlauth_host and mail_attribute_dict.

IMAP Hibernation

WARNING

This is not supported on kqueue based systems currently, such as FreeBSD.

Dovecot supports moving connections that have issued IDLE to a special holding process, called imap-hibernate. This process is responsible for holding the idle processes until they need to be thawed.

Configuration

imap_hibernate_timeout specifies the delay before moving users to imap-hibernate process. This requires inter-process communication between imap and imap-hibernate process.

imap_hibernate_timeout = 5s

service imap {
  # Note that this change will allow any process running as
  # $default_internal_user (dovecot) to access mails as any other user.
  # This may be insecure in some installations, which is why this isn't
  # done by default.
  unix_listener imap-master {
    user = $default_internal_user
  }
}

# The following is the default already
service imap {
  extra_groups = $default_internal_group
}
service imap-hibernate {
  unix_listener imap-hibernate {
    mode = 0660
    group = $default_internal_group
  }
}

How it Works

When client issues IDLE, the connection socket is moved to the hibernation process. This process is responsible for keeping all connections that are idling, until they issue some command that requires them to be thawed into a imap process. This way, memory and CPU resources are saved, since there is only one hibernation process.