/etc/dovecot/dovecot-ldap.conf.ext

# server
hosts = 127.0.0.1
ldap_version = 3

# 
dn = cn=admin,dc=example,dc=org
dnpass = test

# auth
auth_bind = yes

#
auth_bind_userdn = uid=%u,ou=People,dc=example,dc=org

# base
base = ou=People,dc=example,dc=org

# user_attrs = homeDirectory=home,uidNumber=uid,gidNumber=gid
user_attrs = homeDirectory=home,uid=user,mail=maildir:/storage/vmails/%d/%n
pass_attrs = uid=user,userPassword=password

dovecot ldif

uid=username,ou=Users,dc=example,dc=org
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: inetOrgPerson
objectClass: posixAccount
mail: username@example.org
cn: username example
sn: example
givenName: username
uid: username
uidNumber: 99
gidNumber: 12
userPassword: test
homeDirectory: /storage/home

error

Apr 23 13:12:39 ns14 dovecot: imap: Debug: Unknown userdb setting: plugin/maildir:/storage/vmails/%d/%n=username@example.org

doveconf -n

# doveconf -n 
# 2.1.17: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-573.22.1.el6.centos.plus.x86_64 x86_64 CentOS release 6.7 (Final) ext4
auth_debug = yes
auth_verbose = yes
first_valid_uid = 99
mail_debug = yes
mail_gid = mail
mail_location = maildir:/storage/vmails/%d/%n
mail_uid = nobody
mbox_write_locks = fcntl
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocols = imap lmtp
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
ssl_ca = </etc/dovecot/certs/ca.crt
ssl_cert = </etc/dovecot/certs/dovecot.crt
ssl_key = </etc/dovecot/certs/dovecot.key
ssl_protocols = !SSLv2 !SSLv3
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocol lmtp {
  mail_plugins = sieve
  postmaster_address = username@example.org
}