|forward in .qmail

hi all

i'm having a problem with the syntax of the .qmail file for realuser
(listed in /var/qmail/control/virtualdomains)

---.qmail---
|preline /usr/local/src/tmda-1.1.9/bin/tmda-filter -S
/home/mydomian/tmda-cgi/stubs/vmailmgr-vdir.sh
|forward "$DEFAULT"@exchangeserver.mydomain.org

---error---
deferral: listvdomain:_unknown_user_'olduser'/preline:_fatal:_unable_to_copy_input:_broken_pipe/

when a delivery is attempted for a unknown user, the last line of the
.qmail states "|forward" but of course there is no message to forward
so it results in a broken pipe error...

the error doesn't impact on the mail server in any way other than the
annoying error logging...

is there a cleaner way to define the syntax in the .qmail file ?