[Dovecot] Postfix - dspam - dovecot deliver fails at deliver

I have Postfix & Dovecot working properly with mail delivered to the
user's ~/Maildir using the postfix config:

mailbox_command = /usr/lib/dovecot/deliver

I would like to insert dspam in between, passing mail from postfix to
dspam and then to dovecot/deliver for delivery.

My objective is to only filter incoming mail. This set up doesn't seem
to be documented anywhere that I've found via Google.

Can someone give me some pointers?

Platform is Ubuntu hardy using Dovecot 1.0.10

• Shawn