[Dovecot] Address Extensions with System Users

[Jeff Grossman]

I am using Dovecot 1.2RC2 with Postfix 2.5.5.  Everything is running fine.
 I would like to start using address extensions and have e-mail
automatically filtered into folders based on the folder name after the "+"
in the e-mail address.  I only use system users, no virtual users.  I have
Deliver configured in main.cf as:

mailbox_command = /usr/local/libexec/dovecot/deliver

Everything I can find about setting this up applies to virtual users which
I do not have.  How do I get Deliver to recognize the address extension. 
I am sure it is easy, and I apologize for not finding the answer out
myself.  Here is a copy of my dovecot -n output:

# 1.2.rc2: /usr/local/etc/dovecot.conf
# OS: Linux 2.6.29-1-amd64 x86_64 Debian squeeze/sid
base_dir: /var/run/dovecot/
protocols: imap imaps pop3 pop3s
ssl_cert_file: /etc/ssl/certs/stikman-godaddy.crt
ssl_key_file: /etc/ssl/private/stikman-godaddy.key
ssl_cipher_list: ALL:!LOW:!SSLv2
disable_plaintext_auth: no
login_dir: /var/run/dovecot//login
login_executable(default): /usr/local/libexec/dovecot/imap-login
login_executable(imap): /usr/local/libexec/dovecot/imap-login
login_executable(pop3): /usr/local/libexec/dovecot/pop3-login
mail_location: maildir:~/Maildir
mail_executable(default): /usr/local/libexec/dovecot/imap
mail_executable(imap): /usr/local/libexec/dovecot/imap
mail_executable(pop3): /usr/local/libexec/dovecot/pop3
mail_plugin_dir(default): /usr/local/lib/dovecot/imap
mail_plugin_dir(imap): /usr/local/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/local/lib/dovecot/pop3
auth default:
  mechanisms: plain login
  passdb:
    driver: pam
  userdb:
    driver: passwd
plugin:
  sieve: sieve