dovecot.conf ---------------------
protocol lda {
  postmaster_address = postmaster@wingstech.it

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


dovecot-sql.conf -------------------------------------------

driver = pgsql
connect = host=127.0.0.1 dbname=postfix user=postfix password=xxxx

user_query = SELECT '/var/mail/' || domain || '/' || local_part AS home, \
 '*:bytes=' || quota AS quota_rule \
 FROM mailbox WHERE (local_part = '%u' OR username = '%u') AND active = true

password_query = SELECT '/var/mail/' || domain || '/' || local_part AS userdb_home, \
        username as user, \
        password, \
        '*:bytes=' || quota AS userdb_quota_rule \
        FROM mailbox WHERE (username = '%u' OR local_part = '%u') AND active = true