[Dovecot] Setting location for indexes

[dovecot.7.signal11 at spamgourmet.com]

We have NFS-mounted user directories which cause significant problems with
stale file handles:

dovecot: IMAP(uid): fstat() failed with file /home/uid/mail/.imap/INBOX/dovecot.index.log: Stale NFS file handle

(a) why can't it just close the file and re-open ?

(b) is there a way to specify with the 'default_mail_env' (or in some other way)
the location of **only** the indexes (and let dovecot autodetect all else?).

I have already tried 'mmap_disable = yes', but that does not solve the problem,
and if I set 'default_mail_env = INDEX=/var/spool/mail/INDEX/%u', I get errors
like:

IMAP(uid): Failed to create storage with data: INDEX=/var/spool/mail/INDEX/uid

I don't want to specify mbox:~/mail/ or mbox:~/Mail/, because some users have
one and some the other.