[Dovecot] imap permission denied

[Roland Schmid]

Hello,

we have problems to access our users mailboxes with SquirrelMail. The
log file /var/log/mail.log says permission denied, only one user is
allowed to read his mail with SquirrelMail.
We are running Debian Sarge, Mailserver is Postfix, imap server is
dovecot. Our Mailboxes are stored in the mbox format. Folder of the mbox
files is /var/mail/ the users are unix system accounts. Normally we read
are mail with the POP-Client Outlook, this works well. But we also want
to be able to read our mail (each user his own mails) via SquirrelMail
/var/log/mail.log says:
Mar 13 10:54:40 webserver dovecot: imap(unix-system-account):
mkdir_parents(/var/mail/.imap/INBOX) failed: Permission denied

In the folder where the mbox files are stored exists 3 folders called
INBOX.drafts
INBOX.sent
INBOX.trash
These folders are owned by the user who is able to read his mail via
SquirrelMail and owned by the group mail. Only the user has read and
write permissions.
Does each user need to have his own INBOX Files?
Could you please help me

Kind regards,
Roland