[Exim] .forward filter and maildir

Does exim's .forward filtering support maildir format?
I've tried using this:
###
if not delivered
then
save $home/.maildir/inbox/
endif
###
in my .forward, but I get this error in exim's msglog:
2003-07-22 22:02:13 /home/jason/.maildir/inbox/ <user@host> R=userforward defer (-30): directory_transport unset in userforward router

Am I doing something wrong, or is this just not supported?