[exim] Check existing user

Hi,

I use exim as a smtp relay. This relay forward the emails to the mail
server. I have the problem that, the relay take all mail and try after
received the email to forward the email to the mail server The problem
is, if a user does not exists and the return-path is false the smtp
relay can not send the bounce message.
In the file /etc/exim4/users i have a list with the allowed users.

i try in the rcpt acl the following

deny domains = lsearch;/etc/exim4/users

but it doesn´t work.

thx

kai