[Exim] quota_warn_message do not send any message

Hi,

trying to send some quota warn message to my users. Think I tried
everything, but there still will be no warning message.

Using exim 4.20 with the transport
virtual_imap_delivery:
driver = appendfile
create_directory
directory_mode = 700
headers_remove = "Bcc"
delivery_date_add
envelope_to_add
return_path_add
user = mail
group = mail
mode = 660
directory = /var/spool/virtual/${domain}/${local_part}/maildir
maildir_format
quota = "${lookup mysql{MYSQL_QUOTA}{${value}M}{20M}}"
quota_warn_threshold = 60%
quota_directory = /var/spool/virtual/${domain}/${local_part}/maildir
quota_warn_message ="To: $local_part@$domain\n\
Subject: Your mailbox\n\n\........"

Debug of transport will print this:
29918 Exim quota = 20971520 old size = 17994285 this message = 677
(included)
29918 file quota = 0 count = 77
29918 writing data block fd=6 size=798 timeout=0
29918 quota threshold = 12582912 old size = 17994285 message size = 798
29918 renaming temporary file
29918 renamed tmp/1061363976.H133989P29918.mx22..... as
new/1061363976.H133989P29918.mx22......
29918 appendfile yields 0 with errno=0 more_errno=0
29914 virtual_imap_delivery transport returned OK for user@???
29914 LOG: MAIN

So it seems to be that exim recognize that the size of the directory is
over threshold, but send no message. ???
Anybody know why?

Thanks

Steffen