[Dovecot] Help with LDA

[Joy]

Hi,
       Can anyone tell me how to specify correctly the following line:-

|/var/qmail/bin/preline -f
/usr/local/dovecot/libexec/dovecot/dovecot-lda -d $USER@$DOMAIN

i want dovecot lda to look for mail address instead of uid which is
without domain in my case.

Thanks