My Test-Env  Dovecot.conf:
==========================

protocols = imap

protocol imap {
   listen = *:143
}

shutdown_clients = yes
log_timestamp = "%Y-%m-%d %H:%M:%S "
syslog_facility = mail
login_log_format_elements = user=<%u> method=%m rip=%r lip=%l %c
login_log_format = %$: %s
mail_location = maildir:~/
mail_nfs_index=yes
mail_nfs_storage=yes
mmap_disable=yes
mail_uid = 3001
mail_gid = 3001
mail_privileged_group = mail
mail_debug = yes
mail_log_prefix = "%Us(%u): "
verbose_proctitle = yes

protocol managesieve {
}

auth_verbose = yes
auth_debug = yes
auth_debug_passwords = yes

auth default {
  mechanisms = login plain digest-md5 cram-md5
  passdb sql {
    args = /etc/dovecot/dovecot-sql.conf
  }

  userdb prefetch {
  }

}

dict {
}

plugin {

}

==============================================


My dovecot-sql.conf
===================

driver = mysql
connect = host=127.0.0.1 dbname=mytest user=my_user password=my_pw
default_pass_scheme = CRYPT

password_query = \
  SELECT username AS user, password, \
    concat_ws('/',home,maildir) AS userdb_home,concat('*:messages=20000000:storage=',quota, 'M') as  userdb_quota_rule, uid AS userdb_uid, gid AS userdb_gid \
  FROM plenty_system_mail_mailbox WHERE username = '%u'

======================
my Maildir
==========

../Maildir/
courierimapkeywords
courierimapuiddb
cur
dovecot.index.cache
dovecot-keywords
dovecot-uidvalidity
new
tmp
courierimapsubscribed
courierpop3dsizelist
dovecot.index
dovecot.index.log
dovecot-uidlist
dovecot-uidvalidity.4e12f2f5
subscriptions
.Privat
.Privat.Wife
.Archive
.Archive.2001