[exim] Custom router problem

Hi all,
I have an error with a custom router that check user quota.
This is my router:

maildir_overquota:
driver = redirect
domains = +local_domains
condition = ${if eq {${readsocket{/var/run/exim_sockd.sock}{QUOTACHECK
${lookup ldap {LDAP_Q_HOMEDIRECTORY} {$value} fail} ${lookup ldap
{LDAP_Q_QUOTA} {$value} fail} $message_size}{3s}{\n}{0}}}{0}{0}{1}}
allow_fail
data = :fail: Mailbox quota exceeded

Whes a user is underquota all is ok, but when a user is overquota I have
as SMTP message: UNKNOWN USER
Why I haven't as SMTP message Mailbox quota exceeded???

This is the extract debug output:

calling maildir_overquota router
rda_interpret (string): :fail: Mailbox quota exceeded
expanded: :fail: Mailbox quota exceeded
file is not a filter file
parse_forward_list: :fail: Mailbox quota exceeded
extract item: :fail: Mailbox quota exceeded
maildir_overquota router forced address failure
----------- end verify ------------
accept: condition test failed
accept: endpass encountered - denying access
SMTP>> 550 unknown user
550 unknown user
LOG: MAIN REJECT
H=domain.it (marco) [1.1.1.1] F=<marco@???> rejected RCPT
user@???: Mailbox quota exceeded

Could you help me?
Thanks

Marco