[Dovecot] sieve fileinto public shared mbox?

[RParr]

Could someone verify if 1) sieve can fileinto shared public mbox? 2) if 
it can, what I might be doing wrong?

I am getting errors in the log when I try to have sieve fileinto a 
shared public mbox.

11/04/10 06:08:12 pm    n3    dovecot    deliver(tarts): sieve: 
msgid=<201011050108.oA5187sm003711 at JaySmith.com>:
    failed to store into mailbox 'public/SPAM/SPAMMM': Mailbox doesn't 
exist: public/SPAM/SPAMMM
11/04/10 06:08:12 pm    n3    dovecot    deliver(tarts): sieve: 
execution of script /home/tarts/.dovecot.sieve failed,
   but implicit keep was successful

----------------------------------------------------
require "include";
require "fileinto";
require "regex";
require "relational";
require "comparator-i;ascii-numeric";

if header :value "ge" :comparator "i;ascii-numeric" "X-Spam-Score" "12"
{
   fileinto "public/SPAM/SPAMMM";
   stop;
}
elsif header :value "ge" :comparator "i;ascii-numeric" "X-Spam-Score" "8"
{
   fileinto "SPAM/MySPAMM";
   stop;
}
elsif header :value "ge" :comparator "i;ascii-numeric" "X-Spam-Score" "4"
{
   fileinto "SPAM/MySPAM";
   stop;
}
-------------------------------------------------------

sieve fileinto user tarts own directories SPAM/MySPAM works fine.

I have defined 2 namespace

namespace private {
    separator = /
    prefix =
    inbox = yes
}

namespace public {
    separator = /
    prefix = public/
    location = mbox:/var/mail/public
    subscriptions = no
}

and I have

mail_access_groups = tarts

The /var/mail/public/SPAM/... folders do exist and user tarts can 
move/copy mail into them just fine; they have permissions as follows:

public/SPAM:
total 28
drwxrws--- 3 mail     tarts 4096 2010-11-04 18:11 ./
drwsrws--- 8 mail     tarts 4096 2010-10-27 18:18 ../
drwxrws--- 5 testuser tarts 4096 2010-11-04 14:49 .imap/
-rw-rw---- 1 testuser tarts 2019 2010-11-04 14:58 SPAM
-rw-rw---- 1 tarts    tarts 2003 2010-11-04 14:58 SPAMM
-rw-rw---- 1 mail     tarts 7175 2010-11-04 18:11 SPAMMM

public/SPAM/.imap:
total 20
drwxrws--- 5 testuser tarts 4096 2010-11-04 14:49 ./
drwxrws--- 3 mail     tarts 4096 2010-11-04 18:11 ../
drwxrws--- 2 testuser tarts 4096 2010-11-04 14:49 SPAM/
drwxrws--- 2 tarts    tarts 4096 2010-11-04 14:49 SPAMM/
drwxrws--- 2 tarts    tarts 4096 2010-11-04 14:49 SPAMMM/

public/SPAM/.imap/SPAM:
total 28
drwxrws--- 2 testuser tarts  4096 2010-11-04 14:49 ./
drwxrws--- 5 testuser tarts  4096 2010-11-04 14:49 ../
-rw-rw---- 1 tarts    tarts 16384 2010-11-04 14:50 dovecot.index.cache
-rw-rw---- 1 testuser tarts   836 2010-11-04 14:58 dovecot.index.log

public/SPAM/.imap/SPAMM:
total 28
drwxrws--- 2 tarts    tarts  4096 2010-11-04 14:49 ./
drwxrws--- 5 testuser tarts  4096 2010-11-04 14:49 ../
-rw-rw---- 1 tarts    tarts 16384 2010-11-04 14:49 dovecot.index.cache
-rw-rw---- 1 tarts    tarts   748 2010-11-04 14:58 dovecot.index.log

public/SPAM/.imap/SPAMMM:
total 32
drwxrws--- 2 tarts    tarts  4096 2010-11-04 14:49 ./
drwxrws--- 5 testuser tarts  4096 2010-11-04 14:49 ../
-rw-rw---- 1 tarts    tarts 17408 2010-11-04 18:03 dovecot.index.cache
-rw-rw---- 1 tarts    tarts  1660 2010-11-04 18:11 dovecot.index.log