[Exim] smarthost problem

Hi folks,
I try to configure exim4 on my linux box to use different
smarthosts based on the envelope-sender-addresses of the incoming
mails waiting for delivery. The clients connect via Outlook and a
normal SMTP connection from the internal network.
So I dropped the following lines into the authenticators section:

cram_md5:
driver = cram_md5
public_name = CRAM-MD5
client_name = "${extract{3}{::}{${lookup{$sender_address}lsearch*{CONFDIR/passwd.client}{$value}fail}}}"
client_secret = "${extract{4}{::}{${lookup{$sender_address}lsearch*{CONFDIR/passwd.client}{$value}fail}}}"

The lines in the file "passwd.client" look like this:
xy@???:localuser:mail.gmx.net:123456:secret

The whole configuration is working as long as the users use lower
case sender adresses. The right smarthost is figured out. But
every time, the user has the idea to use mixed-case-adresses the
message won`t get deliverd because the lookup of the login data fails.
Can anyone give me a hint how to handle this problem? I tried to
use the "lc" option, but I was not able to get things running.

Thanks for your help.

Thorsten.