[exim] system filter

Hiya

Im trying trying to get the following to work.

if $sender_host_name is localhost then
     noerror unseen save 
/usr/local/backups/mail/${substr_0_10:$tod_log}/${lc:$local_part}/outgoing/$message_exim_id
else
     noerror unseen save 
/usr/local/backups/mail/${substr_0_10:$tod_log}/${lc:$local_part}/incoming/$message_exim_id
endif

I seem to be having a problem with ${lc:$local_part}

The following is in my access log.

/usr/local/backups/mail/2010-04-14//incoming/1O21SI-0007EO-QK
<system-filter> T=address_file

Would anyone know how I could get the string before @.

Kind Regards
Brent Clark