Postfix - Auth Dovecot Backend

smtpd_sasl_authenticated_header = yes
smtpd_sasl_auth_enable = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth

 auth default {
  mechanisms = plain login

 socket listen {
    client {
      path = /var/spool/postfix/private/auth
      mode = 0660
      user = postfix
      group = postfix
    }
  }
}