[Dovecot] .temp file, public folders

[Mayer]

We recently migrated from UW-Imap to dovecot.
Our setup:
dovecot 1.2.6
postfix 2.3.3 (from rhel)
systemusers without shell access to the mailstore
location = maildir:/maildir/%1n/%u:LAYOUT=fs
All mails on local disks.

Performance is great compared with mbox. But I still have some 
unresolved problems, mainly
concerning deliver.

Deliver sometimes wants to write a  .temp.<hostname>.some-number file in 
the  user's home
directory.  If the user is over quota (in his home!) he can't receive 
mails though no mails are
stored in his home. The user homes are on a distributed filesystem 
(AFS). The mails are
stored locally on the mailserver as I mentioned before.
How can I tell deliver in the configuration to write the temp file at 
another location ?

We would like to offer public folders for work groups. Reading with imap 
works fine.
But I didn't find a way to tell deliver to drop mails directly in a 
public folder.
Should I use Sieve instead ?
Maybe this is rather a postfix question. Postfix is calling deliver. 
Userdb in the dovecot
config is passwd. Postfix looks for local users in:
local_recipient_maps = $alias_maps $recipient_canonical_maps 
unix:passwd.byname
Any help is appreciated.

Helga Mayer