[Exim] virtual_delivery not working correctly

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]

Dear All,

I am using the following virtual_delivery director to lookup mailboxes via MySQL however when no mailbox is returned exim still attempts delivery.
Is there a way to make Exim not to perform delivery if the user or data was not found/selected from the $lookup ?

MYSQL_MAILBOX = SELECT mailboxfile FROM mailboxes WHERE username = '${quote_mysql:$local_part}'

virtual_delivery:
driver = appendfile
file = ${lookup mysql {MYSQL_MAILBOX}{$value}}
delivery_date_add
envelope_to_add
return_path_add
user = mail
group = mail
#mode = 0660

Thanks

Paul
--