Cannot subscribe to Public mailbox

[MI]

Some progress was achieved by using symlinks to make the Maildir a subdirectory of 
the public namespace location:

    mkdir /home/others/sharedmail/public-mails

    ln -si /home/others/sharedmail/Maildir/ /home/others/sharedmail/public-mails/.Public

and

    namespace {
           list = yes
           location = maildir:/home/others/sharedmail/public-mails
           prefix = Public.
           separator = .
           subscriptions = no
           type = public
    }


However, Postfix delivers new mails with mode 600:

    # ls -l /home/others/sharedmail/Maildir/cur
    -rw------- 1 sharedmail sharedmail 1596 Sep  4 16:25
    1472999138.V804I3c0028M764419.pe2:2,
    -rw------- 1 sharedmail sharedmail 1596 Sep  4 16:36
    1472999809.V804I3c002cM121024.pe2:2,

So I can now subscribe to the Public maildir, but still cannot read the mails...

I had the impression that using dovecot's lmtp delivery could help, but when I set 
"mailbox_transport = lmtp:unix:private/dovecot-lmtp" in postfix main.cf, mails are 
rejected because my recipient maps / virtual_alias maps are ignored.

Would someone know what I have to do? I could (re-)try configuring dovecot ACLs, but 
according to http://wiki2.dovecot.org/SharedMailboxes/Permissions it looks like it 
wouldn't help:

    "If you use more than one UNIX UID <http://wiki2.dovecot.org/UserIds> for your
    mail users (e.g. you use system users), you'll need to make sure that all users
    can access the mailboxes on filesystem level. (ACL plugin
    <http://wiki2.dovecot.org/ACL> won't help you with this.) "


Basically, I have:

- email users are system users
- Postfix holds the email -> user maps (mostly in regexp or pcre format)

How can I have Public or Shared mailboxes?



-------- Original Message --------
> I'm trying to set up a Public mailbox, but when I try to subscribe to it in 
> Thunderbird, nothing happens: it doesn't appear as a folder, and when I go back to 
> the Subscribe window, it is unchecked again.
>
> I use postfix local dlivery, which created a Maildir when a mail arrived for this 
> mailbox: /home/others/sharedmail/Maildir/{cur,new,tmp}
>
> (sharedmail is a system user so that Postfix can deliver the mail; it doesn't have 
> a login shell or a password)
>
> To make sure it is not a simple filesystem permissions problem, I did "chmod -R 
> a+rwX Maildir/"
>
> This is my current config ( I tried to keep it short by leaving only what might be 
> relevant ):
>
>    # dovecot -n
>    # 2.1.7: /etc/dovecot/dovecot.conf
>    # OS: Linux 3.2.0-4-amd64 x86_64 Debian 7.9
>    ...
>    mail_location = maildir:~/Maildir
>    namespace {
>       list = yes
>       location = maildir:/home/others/sharedmail/Maildir
>       prefix = Public.
>       separator = .
>       subscriptions = no
>       type = public
>    }
>    namespace inbox {
>       inbox = yes
>       location =
>       mailbox Drafts {
>         special_use = \Drafts
>       }
>       mailbox ...
>         ...
>       }
>       prefix =
>       separator = .
>    }
>    passdb {
>       driver = pam
>    }
>    plugin {
>       sieve = ~/.dovecot.sieve
>       sieve_dir = ~/sieve
>    }
>    protocols = " imap"
>    service auth {
>       unix_listener /var/spool/postfix/private/auth {
>         group = postfix
>         mode = 0660
>         user = postfix
>       }
>    }
>    userdb {
>       driver = passwd
>    }
>
>
> Also, I'm not sure about the difference between type=public and type=shared. But 
> anyway, I tried both with the same result.