[Dovecot] Error on mailbox file

[augustocasagrande at gmail.com]

Hi!

I'm running Postfix 2.2.9 as MTA. I have several mail clients , such as  
Outlook 2000/xp/2007. The clients comunicate to the MTA using pop3/smtp  
protocols.
I have also installed Webmail Squirrel for the internet users. Squirrel  
needs an Imap server, so i have utilized Dovecot 1.2.6.
To authenticate users , i use pam_winbbind to validate users to my NT  
domain.

Here is my problem :
Today i checked my /var/log/mail log and this took my attention :

Sep 29 08:25:07 host1 dovecot: imap-login: Login: user=<USER>,  
method=PLAIN, rip=172.20.22.12, lip=172.20.22.14
Sep 29 08:25:07 host1 dovecot: IMAP(user): mail_location: mbox:  
mkdir(/home/NT-DOMAIN/user/mail) failed: Permission denied  
(euid=10188(user) egid=10001(domain users) missing +w perm: /home)
Sep 29 08:25:07 host1 dovecot: IMAP(user): Fatal: Namespace initialization  
failed

Here , i assume the user put his user name in CAPS (USER), and that i  
generating an error. Then Dovecot tries to create the directory  
/home/NT-DOMAIN/user/mail. In my dovecot.conf file , i have :
mail_location = mbox:~/mail:INBOX=/var/mail/%u

Now , the log continues :
Sep 29 08:25:18 host1 dovecot: imap-login: Login: user=<user>,  
method=PLAIN, rip=172.20.22.12, lip=172.20.22.14
Sep 29 08:25:18 host1 dovecot: IMAP(user): mail_location: mbox:  
mkdir(/home/NT-DOMAIN/user/mail) failed: Permission denied  
(euid=10188(user) egid=10001(domain users) missing +w perm: /home)
Sep 29 08:25:18 host1 dovecot: IMAP(user): Fatal: Namespace initialization  
failed

Here the user re-enters the user, now correctly, but the problem persists .

Finally , a could see this new log entry :
Sep 29 10:50:43 host1 postfix/local[19663]: 50B98441E0:  
to=<user at domain.com>, orig_to=<user at domain.com>, relay=local, delay=0,  
status=bounced (cannot access mailbox /var/mail/user for user user. cannot  
open file: Permission denied)

Here some user tries to send an email to this user , and a permission  
denied error occurs.

Watching the permissions on the file :
-rw------- 1 user users 1499655 2010-09-29 02:18 /var/mail/user

Maybe some Dovecot process is blocking the file access ?

List of Dovecot process:
ps ax | grep dovecot
25997 ? Ss 0:07 /usr/sbin/dovecot
15399 ? S 0:02 dovecot-auth
20614 ? S 0:00 dovecot-auth -w

How can i fix this issue?

Thank you very much !
Augusto