[exim] quota warn problem?

Why my users can't recieve a mail warn message.
here is my setup.

MSG_QUOTA = "\ To: $local_part@$domain\n\
                        Subject: Mailbox quota warning\n\n\
                        This message was automatically generated by the mail delivery software.\n\n\
                        You are now using over 90% of your allocated mail storage quota.\n\n\
                        If your mailbox fills completely, further incoming messages will be automatically\n\
                        returned to their senders.\n\n\
                        Please take note of this and remove unwanted mail from your mailbox.\n"

virtual_delivery:
  driver = appendfile
  maildir_format = true
  create_directory = true
  directory_mode = 0770
  directory = /var/maildirs/$domain/$local_part/Maildir
  delivery_date_add
  envelope_to_add
  group = nobody
  user = mailnull
  mode = 0660
  quota = ${lookup mysql{select opt_quota from Auth \
                where username = '${quote_mysql:$local_part}' \
                and domain = '${quote_mysql:$domain}'}{${value}M}}
  quota_warn_threshold = 90%
  quota_is_inclusive = true
  quota_warn_message = MSG_QUOTA
#  maildir_tag = ,S=$message_size
#  quota_size_regex = ,S=(\d+):
#  maildir_use_size_file = false
  no_mode_fail_narrower
  return_path_add

2006-05-17 18:07:47 1FgN0e-000D0U-0G == ivo@??? R=virtual_user T=virtual_delivery defer (-22): mailbox is full (MTA-imposed quota exceeded while writing to tmp/1147878467.H469808P50210.DeViLsPoT.com)