[Dovecot] Expire Plugin

[e-frog]

I was struggling for a while to get the expire plugin to expunge mail. I 
was using (and still would like to use) PAM as passdb and passwd as 
userdb but with this I could not get the expire-tool to delete any 
mails. For debugging purpose I switch to passwd-file as userdb/passdb 
and enabled mail_debug=yes.
        
1. passwd-file contains home directory and userdb_mail (->working!)

/etc/dovecot/dovecot.users:
foo:{plain}xxxx:2004:100::/home/foo::userdb_mail=maildir:~/Maildir
        
root at mail:# dovecot --exec-mail ext 
/usr/local/libexec/dovecot/expire-tool --test
Info: maildir: data=~/Maildir
Info: maildir++: root=/home/foo/Maildir, index=, control=, 
inbox=/home/foo/Maildir
Info: Trash: seq=1 uid=2586: Expunge
Info: Trash: timestamp 1220392046 -> 1220451447

2. passwd-file contains home directory only (->not working!)

/etc/dovecot/dovecot.users:
foo:{plain}xxxx:2004:100::/home/foo

root at mail:# dovecot --exec-mail ext 
/usr/local/libexec/dovecot/expire-tool --test
Info: maildir: data=/root/Maildir
Info: maildir++: root=/root/Maildir, index=, control=, inbox=/root/Maildir
Error: stat(/root/Maildir/.Trash/tmp) failed: Permission denied 
(euid=2005 egid=100)

The expire-tool doesn't find foo's mailbox even if mail_location is set 
global in the configuration file (please see below). This is exactly the 
same result as when using PAM and passwd. So I guess there is something 
wrong with user lookup or setting/expanding HOME and MAIL in the 
expire-tool.

root at mail:# dovecot -n
# 1.1.3: /etc/dovecot/dovecot.conf
log_path: /var/log/mail/dovecot.log
info_log_path: /var/log/mail/dovecot.log
ssl_disable: yes
disable_plaintext_auth: no
login_dir: /var/run/dovecot/login
login_executable: /usr/local/libexec/dovecot/imap-login
login_process_size: 16
max_mail_processes: 16
mail_location: maildir:~/Maildir
mail_debug: yes
mail_process_size: 64
mail_plugins: quota imap_quota expire
auth default:
  passdb:
    driver: passwd-file
    args: scheme=plain /etc/dovecot/dovecot.users
  userdb:
    driver: passwd-file
    args: /etc/dovecot/dovecot.users
  socket:
    type: listen
    client:
      path: /var/run/dovecot/auth-client
      mode: 432
      user: dovecot
      group: mail
    master:
      path: /var/run/dovecot/auth-master
      mode: 384
plugin:
  quota: maildir
  quota_rule: *:storage=1GB
  quota_rule2: Trash:storage=10%%
  expire: Trash 1 Junk 1
  expire_dict: proxy::expiredict
dict:
  expiredict: db:/var/lib/dovecot/expire.db