[Exim] mysql lookup

I've upgrade exim 3 to exim 4.30.
Users are stored in a mysql database.
Everything works fine, except a little problem that i can't resolve.
When a message it's sent to an existent user, exim returns the follow
message to the sender:

[...]
 unknow_user@???
    Failed to expand group string "${lookup mysql{ SELECT gid FROM users
WHERE id='${local_part}'}{$value}fail}" for the maildir_home
transport:
 "lookup" failed and "fail" requested: retry timeout exceeded

How can i change this to anything else like "Delivery to the following
recipients failed - unknow_user@???"?

Thanks
Paulo


[MY ROUTER]
local_user:
debug_print = "R: local_user for $local_part@$domain"
driver = accept
transport = maildir_home

[MY TRANSPORT]
maildir_home:
  debug_print = "T: maildir_home for $local_part@$domain"
  driver = appendfile
  directory = ${lookup mysql{ SELECT maildir FROM users \
                WHERE id='${local_part}'}{$value}fail}
  user = ${lookup mysql{ SELECT uid FROM users \
                WHERE id='${local_part}'}{$value}fail}
  group = ${lookup mysql{ SELECT gid FROM users \
                WHERE id='${local_part}'}{$value}fail}
  delivery_date_add
  envelope_to_add
  return_path_add
  maildir_format